A  HANDBOOK 

OF 

COLLOID-CHEMISTRY 


OSTWALD 


V*" 


(Frontispiece) 


A  HANDBOOK 

OF 

COLLOID-CHEMISTRY 

THE  RECOGNITION  OF  COLLOIDS,  THE 
THEORY  OF  COLLOIDS,  AND  THEIR 
GENERAL  PHYSICO-CHEMI- 
CAL PROPERTIES 


BY 
DR.  WOLFGANG  OSTWALD 

PRIVATDOZENT  IN  THE  UNIVERSITY  OF  LEIPZIG 

FIRST  ENGLISH  EDITION 
TRANSLATED  FROM  THE  THIRD  GERMAN  EDITION 

BY 

DR.  MARTIN  H.  FISCHER 

PROFESSOR   OF   PHYSIOLOGY  IN  THE  UNIVERSITY  OF  CINCINNATI 

WITH  THE  ASSISTANCE  OF 

RALPH  E.  OESPER,  PH.  D.      AND     LOUIS  HERMAN,  M.  D, 

INSTRUCTOR  IN  CHEMISTRY,  NEW  YORK  STAFF  PHYSICIAN,  MOUNT  SINAI 

UNIVERSITY  HOSPITAL,  NEW  YORK 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT   STREET 


9 


COPYRIGHT,  1915,  BY  P.  BLAKISTON'S  SON  &  Co, 


TH.TC    MAP  UK     PKKSS     Y  O  H  K.     J'A 


TRANSLATOR'S  PREFACE 

The  day  is  past  when  the  importance  of  colloid-chemistry  to 
the  worker  in  the  abstract  or  applied  branches  of  science  needs 
emphasis.  The  endeavor  of  the  "pure"  chemist  to  reduce  all 
substances  to  crystalloid  form  and  from  the  knowledge  of  their 
behavior  to  resynthesize  the  phenomena  of  nature  has  been  a 
good  one,  but  the  limitations  of  such  a  point  of  view  have  grown 
daily  more  apparent.  It  happens  that  nature  has  chosen  the 
colloid  form  in  which  to  show  her  face.  Crystalloid  behavior  is 
the  exception,  colloid  behavior  the  rule,  in  the  cosmos.  Whether 
we  deal  with  the  regions  above  the  earth,  as  the  color  of  sky,  the 
formation  of  fogs,  the  precipitation  of  rain  and  snow,  or  with  the 
earth  itself  in  its  muddied  streams,  its  minerals  and  its  soils,  or 
with  the  molten  materials  that  lie  under  the  earth,  the  problems 
of  colloid-chemistry  are  more  to  the  fore  than  have  ever  been  the 
crystalloid  ones. 

To  the  abstract  thinker  in  science  colloid-chemistry  there- 
fore, because  of  its  universality,  represents  the  larger  field. 
But  the  practical  worker  knows,  too,  that  in  a  better  knowledge 
of  the  properties  of  those  very  materials  which  the  orthodox 
chemist  has  too  often  cast  aside  in  his  jellies,  pastes  arid  glues,  is 
found  the  explanation  of  so  much  that  interests  him.  Is  it  any 
wonder  then  that  colloid-chemistry  appeals  to  the  agriculturalist, 
the  metallurgist,  the  dealer  in  precious  stones,  the  tanner  of  skins, 
the  manufacturer  of  wood  pulps  and  paper,  the  dyer,  the  his- 
tologist,  the  steel  worker,  the  weaver  of  textiles,  the  smelter,  the 
manufacturer  of  paints? 

Not  only  the  inorganic  world  but  the  organic  also  has  chosen 
the  colloid  realm  in  which  to  manifest  itself.  Living  matter, 
whether  of  plants  or  animals,  and  under  normal  or  pathological 
conditions,  is  chemistry  in  a  colloid  matrix;  whence  colloid- 
chemistry  comes  to  concern  every  botanist  and  zoologist,  the 


3375S8 


/ 
vi 


physiologist,  the  pathologist  and  the  practical  man  in  medicine 
and  surgery. 

Under  the  circumstances,  does  this  volume,  known  the  world 
over  as  the  authoritative  and  classical  text,  need  an  introduction 
to  any  of  our  people  who  think  in  the  day's  work?  It  can  only 
seem  somewhat  strange  that  three  large  German  editions  and 
seven  years  were  required  before  its  first  issue  in  the  tongue  of 
Thomas  Graham  and  the  brilliant  modern  group  of  English- 
speaking  colloid-chemists.  Wolfgang  Ostwald's  writings  repre- 
sent in  colloid-chemistry  what  those  of  Charles  Gerhardt  represent 
in  organic,  Justus  Liebig  in  agricultural,  and  Wilhelm  Ostwald 
in  physical  chemistry. 

MARTIN  H.  FISCHER. 

EICHBERG  LABORATORY  OF  PHYSIOLOGY, 

UNIVERSITY  OF  CINCINNATI. 


TABLE  OF  CONTENTS 


PRACTICAL  INTRODUCTION 

PAGE 
§i.  Identification  of  Colloid  Systems  by  Elementary  Methods: 

1.  General  Considerations i 

2.  The  Colloid  State  is  Independent  of  Chemical  Composition.    ...       2 

I.  ELEMENTARY  GENERAL  COLLOID  ANALYSIS 

3.  Chemically  Homogeneous  and  Heterogeneous  Liquids 3 

4.  True  Solutions,  Mechanical  Suspensions  and  Colloid  Solutions.    .    .  4 

5.  The  Properties  of  Mechanical  Suspensions 4 

6.  The  Instability  of  Mechanical  Suspensions 5 

7.  Differentiation  of  a  True  from  a  Colloid  Solution .  6 

8.  The  Tyndall  Phenomenon 7 

9.  The  Distinction  of  True  from  Colloid  Solutions  on  the  Basis  of 
Their  Mechanical  Properties 9 

10.  Dialysis  Experiments 10 

u.  Transition  Systems 12 

II.  ELEMENTARY  SPECIAL  COLLOID  ANALYSIS 

12.  Suspensoids  and  Emulsoids 12 

13.  Viscosity 13 

14.  Coagulation 13 

15.  Influence  of  Concentration .  14 

1 6.  The  Electric  Properties  of  Colloids 14 

17.  The  Mutual  Precipitation  of  Colloids 16 

18.  Electrophoresis 16 

19.  Summary 16 

PART  I 
GENERAL  COLLOID-CHEMISTRY 

CHAPTER  I 
THE  GENERAL  CONSTITUTION  OF  COLLOID  SYSTEMS 

§2.  The  Colloids  as  Heterogeneous  Systems: 

1.  The  Concept  of  Heterogeneity 21 

2.  Physical  and  Chemical  Heterogeneity 22 

§3.  Colloids  as  Disperse  Heterogeneous  Systems: 

i.  The    Phases  are  in  Contact  with  Each  Other  under  Conditions 

vii 


Vlll  TABLE    OF    CONTENTS 

PAGE 

Which  Permit  the  Development  of  Much  Surface  between  Them.  23 
2.  The  Phases  are  so  Distributed  within  the  System  That  Externally 

the  Whole  Appears  Homogeneous 23 

§4.  The  Disperse  Phase  and  the  Dispersion  Means 25 

§5.  Specific  Surface  in  Dispersoids;  Degree  of  Dispersion 26 

§6.  Classification  of  the  Dispersoids  According  to  Their  Degree  of  Dispersion .  29 

1.  Classification  of  Zsigmondy 29 

2.  Classification  of  Dispersoids  According  to  Their  Degree  of  Dis- 
persion       31 

3.  Defects  of  this  Principle  of  Classification. 34 

4.  Polydispersoids 35 

5.  Dispersoids  Varying  with  Changes  in  Concentration 35 

6.  Temperature-variable  Dispersoids 36 

7.  Complex  Dispersoids 36 

8.  Transition  Phenomena 39 

§7.  General  Colloid-chemical  Nomenclature 40 

CHAPTER  II 

RELATIONS  BETWEEN  THE  PHYSICAL  STATE  AND  THE  GENERAL 
PROPERTIES  OF  COLLOID  SYSTEMS 

§8.  Classification  of  Dispersoids  According  to  the  States  of  Their  Phases: 

1.  The  Physical  State  of  the  Disperse  Phases  as  a  Principle  of  Classi- 
fication   42 

2.  Classification  of  the  Dispersoids  According  to  the  Physical  State 

of  Their  Phases 43 

§9.  Transition  Phenomena.     Complex  Dispersoids 44 

1.  General   Considerations.     Influence  of  Temperature   and   Degree 

of  Dispersion 44 

2.  Influence  of  Concentration  upon  State  in  Complex  Dispersoids   .    .  45 

A.  COMPLEX  SYSTEMS  HAVING  THE  COMPOSITION  LIQUID  +  LIQUID 

(a)  Influence  of  Concentration  upon  the  State  of  the  Dispersoid 

as  a  Whole 47 

(b)  Influence  of  Concentration  on  the  State  of  the  Disperse  Phase  48 

B.  COMPLEX  DISPERSOIDS  HAVING  THE  COMPOSITION  LIQUID  +  SOLID 

(a)  Influence  of  Concentration  on  the  State  of  the  Dispersoid 

as  a  Whole 48 

(&)  Influence  of  Concentration  on  the  State  of  the  Disperse  Phase.  49 

§10.  Colloid  Systems  as  Suspensoids  and  Emulsoids 49 

1.  General  Considerations 49 

2.  The  Empirical  Establishment  of  Two  Classes  of  Colloids 50 

3.  The  Theoretical  Characterization  of  the  Two  Classes  of  Colloids   .  51 

4.  The  Frequency  of  Occurrence  of  Complex  Emulsoids 53 

5.  Relation  of  These  Two  Colloid  Classes  to  Molecular  Dispersoids.    .  54 

6.  Suspensoids  and  Emulsoids 54 

§n.  Transition  Phenomena  between  Suspensoids  and  Emulsoids 55 

§12.  The  Crystalline  (Vectorial)  Constitution  of  the  Disperse  Phase 56 

1.  The  Concept  of  Crystallinity 56 

2.  Direct  Proof  of  Crystallinity  in  Colloids 58 


TABLE   OF   CONTENTS  IX 

PAGE 

3.  Indirect  Proof  for  the  Crystallinity  of  Colloid  Phases 58 

4.  Dependence  of  Crystallinity  upon  Size  of  Particles 61 

5.  Crystallinity  of  Emulsoids 64 

CHAPTER  III 
GENERAL  ENERGETICS  OF  THE  DISPERSOIDS 

§13.  Surface  Energies 66 

1.  Forms  of  Energy  Characteristic  of  Dispersoids 66 

2.  Surface  Energy  of  the  First  Order 66 

3.  Surface  Energy  of  the  Second  Order 67 

4.  The  Relation  of  Surface  Energy  of  the  Second  Order  to  Other 
Forms  of  Energy 71 

§14.  Dependence  of  Surface  Energies  upon  Specific  Surface 72 

1.  General  Considerations 72 

2.  Surface  Energy  of  the  First  Order  and  Specific  Surface 74 

3.  Surface  Energy  of  the  Second  Order  and  Specific  Surface 74 

4.  Dependence  of  Surface  Tensions  upon  Specific  Surface 76 

§15.  Reciprocal  Effects  of  the  Two  Surf  ace  Energies 77 

1.  General  Considerations 77 

2.  Discontinuous  Increase  in  Surface 78 

3.  Theory  of  Dispersion 80 

4.  Consequences  of  the  Energetic  Theory  of  Dispersion 82 

5.  Discontinuous  Diminutions  in  Surface 84 

6.  Theory  of  Condensation 88 

§16.  Influence  of  the  Specific  Surface  upon  the  Relations  between  Surface 

Energies  and  Other  Forms  of  Energy 91 

1.  Specific  Surface  and  Volume  Energy;  Capillary  Pressure 91 

2.  Specific  Surface  and  Changes  of  State 91 

3.  Specific  Surface  and  Electrical  Energy '.''.  92 

4.  Specific  Surface  and  Chemical  Energy 93 

5.  Specific  Surface  and  Radiant  Energy 97 

CHAPTER  IV 

DISTRIBUTION  OF  THE  COLLOID  STATE  AND  THE  CONCEPT  OF  COLLOID 

CHEMISTRY 

§17.  The  Fundamental  Independence  of  the  Colloid  State  of  the  Chemical 

Nature  of  the  Phases 99 

1.  Statistical  and  Experimental  Development  of  the  Idea  of  the 
Universality  of  the  Colloid  State 99 

2.  Universality  of  the  Colloid  State  as  a  Necessary  Consequence  of 
Characterizing  Colloid  Solutions  as  Disperse  Systems 101 

§18.  Isocolloids 102 

§19.  Multiplicity  of  the  Colloid  State  of  One  and  the  Same  Substance.     Ex- 
ample: Colloid  Ice 106 

i.  Isocolloids  of  H2O 107 


TABLE    OF   CONTENTS 

PAGE 

2.  Chemically  Heterogeneous  H2O  Colloids 109 

The  Concept  of  Colloid-chemistry .in 


PART  II 
SPECIAL  COLLOID-CHEMISTRY 

CHAPTER  V 
MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 

I.  RELATIONS  OF  VOLUME  AND  MASS  IN  COLLOIDS 

§21.  Volume  and  Density  Relations  in  Colloids 115 

1.  Volume  Relations  of  Colloid  Systems 115 

2.  Density  and  Space  Relations  in  Colloid  Systems 120 

3.  The  Concentration  Function  of  Density  in  Colloid  Systems.    ...  124 

4.  Thermal  Coefficient  of  Expansion  in  Colloids 126 

§22.  Vapor  Tension,  Boiling  Point  and  Freezing  Point  of  Colloid  Solutions.    .  128 

1.  General  Remarks 128 

2.  Measurements  of  Vapor  Pressure  of  Colloid  Solutions 129 

3.  Elevation  of  Boiling  Point  of  Colloid  Solutions 130 

4.  Depression  of  Freezing  Point  of  Colloid  Solutions 131 

§23.  Mass-relations  in  Colloids 132 

1.  Concentration  of  Colloid  Systems 132 

2.  Experimental  Work  on  Saturation  in  Colloid  Solutions 134 

3.  Theoretical  Considerations  Bearing  on  the  Saturation  of  Colloids  .  136 

4.  Supersaturation  in  Colloid  Systems 138 

§24.  Molecular  Weight  of  Substances  in  the  Colloid  State  as  Measured  by 

Changes  in  the  Constants  of  the  Dispersing  Medium 140 

1.  General  Remarks 140 

2.  Examples  of  the  "Molecular  Weights"  of  Substances  in  the  Colloid 
State  as  Determined  by  Changes  in  the  Constants  of  the  Dispersing 
Medium 142 

II.  INTERNAL  FRICTION  AND  SURFACE  TENSION  OF  COLLOIDS 

§25.  Internal  Friction  of  Colloid  Systems 145 

1.  General  Remarks 145 

2.  Internal  Friction  of  Suspensoids 146 

3.  Effects  of  External  Conditions  upon  the  Viscosity  of  Suspensoids.  150 

4.  Mechanical  Theory  of  the  Viscosity  Relations  in  Suspensoids  .    .    .  152 

5.  Viscosity  of  Emulsoids.    Literature 153 

6.  Viscosity  Changes  in  Emulsoids  with  Time 154 

7.  Effect  of  Mechanical  Treatment  on  Viscosity  of  Emulsoids  .    .    .    .158 

8.  Influence  of  "Inoculation"  on  Viscosity  of  Emulsoids 158 

9.  Influence  of  Thermal  History  on  Viscosity  of  Emulsoids 159 

10.  Influence  of  Concentration  on  Viscosity  of  Emulsoids 161 

11.  Influence  of  Temperature  on  Viscosity  of  Emulsoids 164 

12.  Influence  of  Added  Substances  on  Viscosity  of  Emulsoids 165 


TABLE    OF    CONTENTS  XI 

PAGE 

13.  Effect  of  Added  Substances  on  Viscosity  of  Emulsoids;  Behavior  of 
Protein  Solutions 169 

14.  Influence  of  Added  Substances  on  Viscosity  of  Emulsoids.     Effects 

of  Non-electrolytes  and  Mixture  of  Dispersing  Media 173 

15.  Viscosity  and  Electrical  Charge  of  Disperse  Phase 174 

16.  Viscosity  and  Degree  of  Dispersion;  Viscosity  of  Coarse  and  Com- 
plex Dispersions 175 

17.  Viscosity  and  Type  of  Disperse  Phase 179 

§26.  Surface  Tension  of  Colloid  Solutions 180 

1.  General  Remarks 180 

2.  Experimental  Facts 181 

CHAPTER  VI 
MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 

III.  MOVEMENT  IN  COLLOID  SYSTEMS  AND  ITS  RESULTS 

§27.  Browian  Movement 186 

1.  General  Remarks 186 

2.  The  Independence  of  Brownian  Movement  of  External  Sources  of 
Energy 189 

3.  More  Exact  Determination  and  Measurement  of  Brownian  Move- 
ment      192 

4.  Uniformity  of  Brownian  Movement 195 

5.  Influence  of  the  Specific  Surface  of  the  Particles 196 

6.  Influence  of  the  Concentration  of  the  Dispersoid 196 

7.  Influence  of  the  Viscosity  of  the  Dispersion  Means 197 

8.  Influence  of  Temperature 198 

9.  Influence  of  Added  Substances 199 

10.  Influence  of  Electrical  Charge 201 

11.  Influence  of  Gravity  on  the  Distribution  of  Vibrating  Particles  .    .  201 

12.  Validity  of  Stokes'  Law  for  Highly  Dispersed  Particles 204 

13.  Kinetic  Theory  of  Brownian  Movement 205 

14.  Determination  of  the  "Molecular  Weight"  of  Dispersed  Particles 
from  Their  Brownian  Movement •. 209 

§28.  Diffusibility  of  Colloids 210 

1.  General  Remarks 210 

2.  Experimental  Study  of  Diffusion  of  Colloids 211 

3.  Experimental  Facts  Regarding  Diffusion  of  Colloids. 213 

4.  Influence  of  Degree  of  Dispersion  on  Diffusion  Velocity 215 

5.  Theory  of  Colloid  Diffusion 217 

6.  Effect  of  Added  Substances  on  Colloid  Diffusion.     Spurious  Diffu- 
sion of  Colloids 219 

§29.  Dialysis  of  Colloid  Systems 222 

1.  General  Remarks 222 

2.  Methods  of  Dialysis 222 

3.  Experimental  Facts  Regarding  the  Dialysis  of  Colloids 224 

4.  Special  Observations  Regarding  the  Dialysis  of  Colloids 227 

§30.  Osmosis  of  Colloid  Systems 231 

i.  General  Remarks  and  Literature 231 


xl  TABLE   OF   CONTENTS 

PAGE 

2.  Methods  of  Measuring  the  Osmotic  Pressure  of  Colloids 233 

3.  Instability  of  Osmotic  Pressure  of  Colloids 235 

4.  Influence  of  Concentration  on  Osmotic  Pressure  of  Colloids.    .    .    .   238 

5.  Influence  of  Temperature  on  Osmotic  Pressure  of  Colloids  ....   242 

6.  Influence  of  Added  Substances  on  Osmotic  Pressure  of  Colloids.    .    244 

7.  On  the  Theory  of  Osmotic  Pressure  of  Colloids 253 

8.  Determination  of  the  "Molecular  Weight"  of  Colloid  Systems  by 
Osmotic  Means 258 

9.  On  the  Moleculo-kinetic  Theory  of  Osmosis  in  Colloid  Systems  .    .    261 
ADDENDUM — OTHER  TYPES  OF  MOVEMENT  IN  DISPERSOIDS 262 

§31.  Filtration  and  Ultrafiltration  of  Colloid  Systems 263 

1.  Filtration  of  Colloid  Systems 263 

2.  Ultrafiltration  of  Colloid  Systems 264 

AUTHOR  INDEX 267 

SUBJECT  INDEX ' 273 


PRACTICAL  INTRODUCTION 


§i.  Identification  of  Colloid  Systems  by  Elementary  Methods 

(The  Elements  of  Qualitative  Colloid-chemical  Analysis) 

i.  General  Considerations. — The  teachings  of  colloid-chem- 
istry are  by  no  means  so  familiar  to  all  who  encounter  colloid 
substances  in  their  scientific  or  practical  work  that  the  questions: 
"How  can  we  recognize  a  colloid?"  or  "When  is  a  body  said  to 
be  a  colloid?"  are  no  longer  raised.  These  questions  have  often 
been  put  to  me,  not  only  by  such  men  of  science  as  physicists, 
physical  chemists,  physicians  and  mineralogists,  but  by  technicians 
who  for  years  perhaps  have  worked  exclusively  in  such  practical 
colloid  problems  as  the  manufacture  of  rubber.  Even  the  organic 
and  inorganic  chemists  frequently  encounter  phenomena,  par- 
ticularly when  they  work  with  highly  polymerized  and  highly 
complex  substances  that  remind  them  of  what  they  know  of  the 
properties  of  colloids,  and  which  make  them  ask  how  they  can  de- 
termine quickly  and  simply  whether  colloid-chemical  principles 
will  help  them  in  the  solution  of  their  problem  or  no.  As  a 
matter  of  fact  I  am  of  the  opinion  that  such  questions  have  not 
been  asked  frequently  enough,  say  in  organic  chemistry,  where 
examination  of  the  colloid  behavior  of  one  and  the  same  organic 
substance  in  different  solvents  would  throw  much  light  on  the 
properties  observed.1  The  youth  of  colloid-chemistry  itself 
justifies  such  questions,  and  their  discussion  is  by  no  means 
either  useless  or  superfluous. 

An  answer  to  the  question:  "How  do  we  know  when  we 

1  We  need  but  call  to  mind  the  modern  problem  of  the  relation  to  each  other  in 
solutions  of  various  kinds  of  color,  chemical  constitution,  molecular  state  and  char- 
acter of  solvent  as  studied  by  A.  Hantzsch  and  his  pupils.  It  seems  to  me  that  a 
colloid-chemical  (dialytic  or  ultramicroscopic)  examination  of  such  variously  colored 
solutions  would  bring  light  especially  in  those  cases  in  which  molecular  weight  deter- 
minations have  been  exhausted  without  result.  The  failure  of  Beer's  law  governing 
the  proportionality  between  thickness  of  layer  and  light  absorption  when  applied  to 
colloids  and  to  solutions  of  dyes,  of  oxime  salts,  organic  ammonium  salts  (see  chapter 
on  optical  properties  in  this  volume)  as  well  as  other  facts  seem  to  me  to  indicate  that 
suitable  colloid-chemical  investigations  in  this  field  will  bring  to  light  as  surprising 
facts  as  did  those  of  J.  Amann  (Koll.-Zeitschr.,  6,  235,  7,  67  (1910)  on  the  colloid  and 
molecular  solubility  of  iodine  in  various  solvents. 


2  'COLLOID-CHEMISTRY 

are  dealing  with  a  colloid?"  would  consist  in  a  presentation  of 
the  elementary  properties  and  the  experimentally  observed  be- 
havior of  colloid  substances.  Such  an  analysis  would  constitute 
the  elements  of  a  qualitative  colloid-chemical  analysis.  A  possible 
method  of  procedure  in  attempting  to  discover  the  colloid  nature 
of  any  substance  is  indicated  in  the  following: 

2.  The  Colloid  State  is  Independent  of  Chemical  Composition. 
— At  first  sight  one  might  hope  to  obtain  information  about  the 
question  under  consideration  by  constructing  a  comprehensive 
table  of  all  the  colloid  substances  or  groups  of  substances  known. 
As  a  matter  of  fact  such  attempts1  have  been  made  even  recently, 
but  never  with  the  full  approval  of  competent  workers  in  the 
field.  It  was  soon  noticed  'that  we  cannot  speak  of  colloid 
substances  in  the  same  way  as  we  (still)  do  of  "liquid-crystalline" 
or  "radio-active"  substances.  We  have  been  compelled  to  rec- 
ognize that  colloid  properties  are  in  no  way  connected  with 
substances  of  definite  chemical  composition  to  the  end  that  only 
certain  elements  or  certain  compounds,  for  example,  appear  as 
colloids.  We  can  speak  of  " colloids "  only  as  we  speak  of  "crys- 
tals," "amorphous"  substances,  "soluble  and  insoluble"  sub- 
stances, or  better  still  of  "gaseous,  liquid,  and  solid"  substances. 
All  substances  can  appear  as  colloids  under  appropriate  conditions. 
This  peculiarity  of  colloid-chemistry,  through  which  it  thus  pre- 
sents itself  not  as  a  study  of  colloid  substances  but  as  a  study  of 
the  colloid  state,  will  be  discussed  in  detail  later.  But  it  is  of 
great  importance  for  even  an  elementary  characterization  of  col- 
loid substances  to  know  that  depending  upon  experimental  con- 
ditions one  and  the  same  chemical  compound  can  appear  either 
as  a  colloid  or  as  a  non-colloid. 

Generally  speaking,  the  knowledge  of  the  chemical  constitu- 
tion of  a  substance  furnishes  no  trustworthy  indication  as  to 
whether  or  not  we  are  dealing  with  a  colloid.  Only  one  law 
has  thus  far  been  deduced  governing  the  relation  between  chemical 
constitution  and  colloid  state:  The  more  complex  chemically  the 
compound,  the  greater  the  probability  that  it  is  in  a  colloid  state. 
Thus  most, of  the  native  proteins  appear  in  a  colloid  state;  and 
the  chemical  composition  of  the  original  colloid,  namely  gelatine, 
is  so  complex  that  we  are  still  largely  ignorant  concerning  it. 
1  See,  for  example,  Koll.-Zeitschr.,  2,  53  (1907). 


PRACTICAL  INTRODUCTION  3 

Solid  and  liquid  and  even  gaseous  bodies  may  appear  in  the 
colloid  state.1  The  liquid  colloids  are  the  most  numerous  and 
the  most  important,  and  thus  far  have  been  most  studied. 
Whenever  we  deal  with  the  class  properties  of  the  colloids  we 
therefore  usually  refer  to  these. 

I.  ELEMENTARY  GENERAL  COLLOID  ANALYSIS 

3.  Chemically  Homogeneous  and  Heterogeneous  Liquids. — 

If  we  wish  to  enquire  into  the  possible  colloid  nature  of  a  given 
liquid,  it  is  well  to  decide  first  whether  it  is  chemically  homoge- 
neous or  chemically  heterogeneous. 

In  an  ideal  case  a  chemically  homogeneous  liquid  has  the 
following  properties:  i.  It  is  susceptible  of  hylo tropic  change, 
that  is,  it  can  be  evaporated  or  frozen  without  changing  its  com- 
position at  any  time  during  the  manipulation.  2.  The  hylo  tropic 
transformations  take  place  within  narrow  limits  of  temperature 
and  pressure;  there  is  only  one  boiling  temperature  and  one 
congelation  temperature;  we  speak  of  melting  and  boiling  points. 
Among  further  properties  of  an  ideal  liquid  is  to  be  mentioned 
the  fact  that  the  temperature  coefficient  of  its  molar  surface  energy 
equals  2.I2.2 

As  is  well  known,  there  are  a  great  number  of  substances,  or- 
ganic liquids,  more  particularly,  which  fulfill  these  requirements 
in  part  only.  They  are  the  mixtures  of  isomeric,  metameriCj  and 
polymeric  substances;  to  which  we  may  add  the  so-called  asso- 
ciated liquids.  Even  though  all  these  liquids  show  the  same 
elementary  analysis  in  every  state  of  aggregation,  yet  they  can 
be  separated  by  fractional  distillation,  for  example,  into  parts 
having  different  boiling  points;  or,  notwithstanding  the  analytic- 
ally identical  composition  of  the  liquid  undergoing  distillation 
and  the  distillate,  it  is  noted  that  the  former  is  not  completely 
evaporated  at  any  definite  temperature.  These  facts  are  illus- 
trated by  the  behavior  of  polymerized  liquids  such  as  styrol- 
metastyrol.  Again,  as  in  associated  liquids,  the  molar  surface 
energy  is  found  to  be  less  than  the  normal. 

The  following  rule  may  be  stated  regarding  the  relation  of 

1  See  Chapter  III  of  this  volume. 

8  See  the  textbook  of  Wilh.  Ostwald,  Grundr.  d.  allgem.  Chemie,  4  Aufl.,  1909  for 
a  discussion  of  the  general  concepts  of  physical  chemistry  employed  here. 


4  COLLOID-CHEMISTRY 

these  properties  to  the  possibility  of  the  appearance  of  a  colloid 
state  in  liquids  of  constant  composition:  The  more  a  liquid 
approaches  the  ideal  of  chemical  homogeneity,  the  less  probable  that 
it  is  in  the  colloid  state.  Therefore,  if  from  general  physico- 
chemical  examination  we  know  a  liquid  not  to  be  " normal"  with 
regard  to  exactness  of  .boiling  point,  molar  surface  energy,  etc.,  it 
is  possible  that  we  are  dealing  with  a  "physical  mixture/'  and 
therefore  with  a  molecular  or  colloid  solution. 

Colloid  liquids  showing  the  same  analytical  composition 
with  every  hylotropic  transformation  are  by  no  means  rare.  Thus 
far  these  have  been  little  studied  from  a  colloid-chemical  point 
of  view.  Details  regarding  the  peculiarities  of  these  so-called 
isodispersoids,  more  especially  the  isocottoids,1  will  be  given  later. 
Nearly  all  the  colloid  solutions  investigated  thus  far  belong  to  the 
class  of  the  chemically  heterogeneous  liquids  discussed  below. 
Since  the  fundamental  properties  of  colloid  liquids  depend,  not 
upon  chemical  composition  but  upon  other  physical  conditions 
which  are  especially  to  be  encountered  in  chemically  heterogeneous 
liquids,  we  shall  also  discuss  in  the  succeeding  paragraphs  the 
general  data  by  means  of  which  we  recognize  the  colloid  character 
of  chemically  homogeneous  liquids. 

4.  True  Solutions,  Mechanical  Suspensions  and  Colloid  Solu- 
tions.— Chemically   heterogeneous   liquids   can   be   separated   by 
changes  in  temperature  and  pressure  (distillation,  freezing,  etc.) 
into  at  least  two  components  of  different  chemical  composition. 
When  we  have  thus  determined  in  our  unknown  that  we  are  deal- 
ing with  a  chemically  heterogeneous  liquid  it  may  appear  in  any 
one  of  three  states : 

(a)  The  unknown  may  be  an  ordinary  or  "true"  (molecular- 
disperse)  solution  of  one  or  more  substances. 

(b)  It  may  be  a  coarse  "mechanical  suspension"  of  one  or 
more  substances  which  form  true  solutions  to  a  limited  degree 
only,  if  at  all. 

(c)  It  may  be  a  colloid  solution. 

5.  The  Properties  of  Mechanical  Suspensions. — In  a  qualita- 
tive analysis  for  the  determination  of  the  "degree  of  dispersion"2 

1  Examples  of  such  liquid  isocolloids  are  oils,  petroleum,  paraffin,  styrol-metastyrol, 
liquid  sulphur  at  temperatures  above  170°,  highly  polymerized  liquids,  etc. 

*  See  page  29  for  a  discussion  of  the  concept  "degree  of  dispersion;"  the  three 
classes  of  systems  mentioned  above  are  distinguished  from  each  other  by  their 
different  degrees  of  dispersion. 


PRACTICAL   INTRODUCTION  5 

in  a  heterogeneous  liquid  the  second  of  the  above  possibilities 
can  be  disposed  of  most  easily.  Typical  "  mechanical  sus- 
pensions" of  substances  but  slightly  soluble  in  liquids,  as  sus- 
pensions of  quartz,  kaolin,  or  oil  in  water,  are  turbid  in  trans- 
mitted light,  and  their  individual  particles  can  be  recognized 
under  the  microscope  (though  sometimes  only  with  high  magni- 
fications and  special  optical  means) . 

If  no  microscope  is  available,  filtration  is  the  next  simplest 
method  by  which  a  suspension  can  be  recognized.  Ordinary 
filter  paper  holds  back  particles  having  a  diameter  greater  than 
about  5/,t;  a  hardened  filter  (Schleicher  and  Schiill,  No.  602  e.h.}9 
those  about  2/z  in  diameter.  Clay  cylinders  and  the  so-called 
Pukall  filters  which  are  frequently  employed  in  bacteriology  will 
even  hold  back  particles  about  0.4  to  o.2/x  in  diameter.1  The 
size  of  the  particles  in  question  can  therefore  be  roughly  measured 
by  the  employment  of  such  differently  permeable  filters.  When 
applied  to  emulsions,  that  is,  suspensions  of  droplets  in  a  liquid, 
.  filtration  is  successful  only  when  the  suspended  droplets  are  not 
materially  deformed  during  filtration.  As  the  investigations  of 
E!  Hatschek2  on  the  filtration  of  emulsions  show,  this  difficulty  does 
not  appear  if  the  droplets  are  moderately  viscous,  as  are  the  droplets 
of  castor  oil  or  olive  oil;  or  when  their  surfaces  are  in  a  condition 
which  gives  the  droplets  themselves  sufficient  stiffness.  Such 
stiffness  may  result  from  the  formation  of  thin  elastic  membranes 
about  the  droplets,  of  the  nature  of  the  well-known  saponin  or 
peptone  films,3  or  it  may  be  due — and  this  seems  most  important— 
to  the  small  size  of  the  droplets  with  its  accompanying  increase 
in  surface  energy.  As  E.  Hatschek  has  shown,  it  is  often  possible 
to  separate  emulsions  into  their  components  by  means  of  appro- 
priate filters. 

6.  The  Instability  of  Mechanical  Suspensions.— Another 
characteristic  of  coarse  suspensions  of  solid  and  liquid  particles 
is  their  instability,  that  is  their  tendency  to  separate  "  spon- 
taneously" into  their  components.  If  we  can  exclude  the  sta- 
bilizing effects  of  additions  of  viscous  substances  such  as  gelatine, 
tragacanth,  etc.,  as  well  as  the  peculiar  " protective  action"  of 

1  For  details  regarding  permeability  and  size  of  pores  in   various  filters    see 
H.  Bechhold,  Zeitschr.  1.  physik.  Chem.,  64,  342  (1908). 

2  See  the  chapter  on  Adsorption  Phenomena  in  Part  III. 

3  E.  Hatschek,  Koll.-Zeitschr.,  6,  254  (1910);  7,  81  (1910). 


6  COLLOID-CHEMISTRY 

small  amounts  of  soap,  saponin,  albumose,  etc.,  separation  occurs 
in  typical  coarse  suspensions  in  accordance  with  the  difference  in 
the  densities  of  their  components.1  Considerable  acceleration  in 
separation  can  be  effected  by  moderately  centrifuging  the  mix- 
ture. A  hand-centrifuge  such  as  is  employed  in  the  study  of 
blood  does  very  well.  The  suspended  component  then  separates 
out,  in  accordance  with  the  difference  in  density,  either  in  the 
form  of  a  precipitate  or  of  a  supernatant  layer.  After  such  a 
separation  has  been  accomplished  either  spontaneously  or  with 
the  assistance  of  a  centrifuge,  the  original  system  can  in 
most  instances  be  restored  by  shaking  the  components  together 
again. 

When  the  suspended  particles  are  in  a  very  finely  divided 
condition,  indefinite  or  negative  results  are  obtained  by  these 
procedures.  Under  such  circumstances  two  possibilities  still 
remain:  the  liquid  in  question  is  either  a  "true"  or  a  "colloid" 
solution. 

7.  Differentiation  of  a  True  from  a  Colloid  Solution. — It  is 
generally  harder  to  distinguish  a  true  from  a  colloid  solution  than 
to  distinguish  a  coarse  suspension  from  either,  yet  this  problem  is 
precisely  the  one  that  arises  most  frequently.  We  must  there- 
fore discuss  the  methods  involved  in  detail. 

A.  Optical  Diferences. — Absolutely  clear  liquids  are  formed 
as  a  rule  by  substances  in  molecular  or  true  solution.  If  a  liquid 
(which  is  not  chemically  homogeneous,  and  which  is  not  a  coarse 
suspension)  is  seen  to  be  turbid,  we  may  suspect  that  it  is  a  colloid 
solution.  The  existence  of  a  slight  turbidity  may  be  recognized 
on  inspection  of  a  rather  thick  layer  of  the  liquid  in  a  thin-walled 
glass  vessel  against  an  opaquely  black  background  (black  paper, 
or  better,  black  velvet).  If  the  liquid  is  colorless  but  turbid, 
the  background  shining  through  it  assumes  a  grayish-white 
appearance.  In  the  case  of  colored  liquids  (in  the  examination  of 
which  it  may  be  necessary  to  employ  a  particular  dilution  or 
thickness  of  layer)  an  optical  effect  appears  which  is  similar  to 
that  observed  on  mixing  water-colors  with  small  quantities  of 
opaque  white  (the  colors  become  milky).  Different  varieties  of 

1  In  this  experiment  it  is  well  to  use  very  long  tubes  and  relatively  "dilute" 
systems.  The  temperature  must  be  kept  constant  to  prevent  ^mixing  of  the  layers 
by  convection  currents.  Closed  basement  rooms  may  be  used  if  a  suitable  thermo- 
stat is  not  available. 


PRACTICAL   INTRODUCTION  7 

nephelometers  have  been  constructed  for  the  more  exact  deter- 
mination of  the  degree  of  turbidity.1 

8.  The  Tyndall  Phenomenon. — A  far  more  delicate  method 
of  demonstrating  the  presence  of  a  very  fine  turbidity  lies  in  the 
use  of  the  so-called  Tyndall  phenomenon.  It  is  well  known  that 
when,  for  example,  the  air  of  a  room  is  intensely  illuminated,  say 
by  sunlight,^from  one  side  only,  dust  particles  are  rendered  visible 
which  cannot  be  seen  when  illumination  is  equal  on  all^sides.  This 
is  the  prototype  of  the  so-called  Tyndall  phenomenon,  the  theory 
of  which  will  be  discussed  later.  Extraordinarily  fine  turbidities 


FIG.  i. — Tyndall  phenomenon. 

can  be  rendered  visible  by  such  means;  in  fact  this  holds  true  to 
such  an  extent  that  special,  measures  become  necessary  if  we  would 
obtain,  for  example,  an  absolutely  "  optically  empty"  distilled  water; 
ordinary  distilled  water  regularly  shows  individual  dust  particles. 

Tyndall  experiments  can  be  best  carried  out  where  sunlight 
and  a  darkened  room  are  available.  The  phenomenon  becomes 
beautifully  evident  if  we  but  let  a  sharply  defined  ray  of  light, 
entering  a  darkened  room  through  a^hole  bored  in  the  shutter 

1  For  simpler  forms  of  such  apparatus  see  H.  von  Oettingen,  Zeitschr.  f.  physik. 
Chem.,33,  i  (1900);  J.  Frie-dlander,  ibid,  38,  430  (1901). 


8  *  COLLOID-CHEMISTRY 

of  a  window,  pass  through  the  liquid  in  question  contained  in  a 
thin-walled  test  tube.  Very  good  results  are  also  obtained  if  a 
projecting  lantern  is  used  from  which  the  light  rays  are  concen- 
trated as  much  as  possible  by  means  of  a  condenser  and  dia- 
phragm (see  Fig.  i).  A  powerful  incandescent  lamp1  enclosed 
in  a  box  that  is  impervious  to  light  and  provided  with  a  small 
opening,  and  if  possible  with  a  condenser,  is  generally  satisfac- 
tory also.  The  thinnest  and  clearest  glass  vessels  must  be 
used.  Special  advantages  are  offered  by  vessels  with  parallel 
walls  which  reflect  light  least.  When  we  work  with  hot  or  very 
cold  liquids  we  use  cotton-stoppered,  double- walled  tubes  from 
which  the  air  has  been  exhausted  (Dewar  tubes).  When  cold 
liquids  are  used  in  these,  the  tubes  may  be  immersed  in  alcohol  to 
avoid  the  condensation  of  water  on  their  outer  walls. 

It  should  now  be  noted  that  it  is  not  the  presence  of  many 
more  or  less  evident  particles  which  may  be  recognized  either 
macroscopically  or  microscopically  that  distinguishes  a  colloid 
from  a  molecular-disperse  (true)  solution.  It  is  rather  the  in- 
tensity of  the  unbroken  light-cone  passing  through  the  solution 
which  betrays  the  state  of  the  liquid.  It  is  safe  to  say  that  liquids 
which  show  no  definite  Tyndall  light-cone  or  show  it  only  in  high 
concentrations  are  molecular-disperse  solutions.  Practically  all 
colloid  solutions  give  a  positive  Tyndall  effect. 

The  Tyndall  phenomenon  is  not  to  be  confounded  with 
fluorescence.  When  a  ray  of  light  enters  many  solutions,  such  as 
those  of  certain  dye-stuffs  and  alkaloids,  the  path  of  the  beam 
betrays  itself  in  brilliant  colors  even  though  these  solutions 
may  not  be  in  the  colloid  state.  The  fluorescence  can  be  dis- 
tinguished from  the  Tyndall  effect  by  looking  at  the  light-cone 
with  a  Nicol  prism.  If  we  look  at  the  Tyndall  cone  of  a  colloid 
solution  through  a  Nicol  prism  we  find  that  it  disappears  when 
the  prism  is  rotated,  to  light  up  again  at  a  certain  angle.  Fluorescent 
light  remains  visible  at  all  angles. 

Emphasis  should  be  laid  on  the  fact  that  the  Tyndall  effect  is 
of  particular  value  in  the  recognition  of  isocolloids. 

It  should  further  be  mentioned  that  the  microscopic  ex- 
amination of  a  Tyndall  cone  with  the  highest  available  magnifi- 

1  Not  only  electrically  lighted  but  gas-lighted  projection  apparatus  as  used  in 
photographic  enlarging,  when  combined  with  a  condenser  is  suited  for  this  purpose. 


PRACTICAL   INTRODUCTION 


cations  at  times  permits  us  to  see  the  individual  particles  which 
in  their  totality  give  rise  to  the  light-cone.  This  is  called  ultra- 
microscopy.  Since  for  ultramicroscopy  special  apparatus  and 
powerful  sources  of  light  are  necessary  which  are  by  no  means 
generally  available,  and  since  the  technique  of  ultramicroscopy 
is  by  no  means  simple,  we  cannot  further  discuss  the  subject 

in  this   elementary   outline   of  colloid    

analysis. 

9.  The  Distinction  of  True  from 
Colloid  Solutions  on  the  Basis  of  Their 
Mechanical  Properties.— 

B.  Mechanical  Differences. — Diffu- 
sion and  dialysis  experiments  provide 
us  with  two  further  simple  methods  for 
distinguishing  molecular-disperse  (true) 
from  colloid  solutions.  These  might 
be  called  the  " classical"  methods  for 
the  qualitative  analysis  of  solutions, 
for  it  was  by  them  that  Thomas  Gra- 
ham in  1 86 1  first  distinguished  be- 
tween the  "states"  of  different  solu- 
tions and  thus  introduced  the  concept 
"colloid." 

(a)  Diffusion  Experiments.- — Per- 
haps the  simplest  and  most  convenient 
experimental  method  of  estimating  the 
diffusion  velocity  of  a  dissolved  substance 
depends  upon  the  fact  that  moder- 
ately concentrated  jellies  of  gelatine, 
agar,  etc.,  offer  only  slight  or  no  resis- 
tance to  the  diffusion  of  substances 
through  them,  as  determined  by  com- 
parison with  the  diffusion  of  these  same 
substances  through  the  pure  solvent. 
For  such  tests  we  prepare  a  5  per  cent. 

gelatine,  or  better  a  2  per  cent,  agar  solution,  fill  some  test 
tubes  about  halfway  with  the  hot  solution,  and  allow  it  to  con- 
geal. It  is  well  to  use  gelatinizing  substances  that  have  been 
thoroughly  washed  and  purified.  The  solution  under  exami- 


FIG.  2. — Diffusion  experi- 
ments with  gelatine  gels  at  end 
of  24  hours.  (a)  (Colloid) 
congo  red;  (b)  (molecularly 
dispersed)  safranin. 


10  COLLOID-CHEMISTRY 

nation  is  then  poured  upon  these  gelatine  or  agar  layers  and  the 
tubes  are  left  standing,  variations  in  temperature  being  avoided 
as  far  as  possible.  A  true  solution  in  water,  either  of  a  dye  such 
as  a  safranin,  or  of  a  colored  salt  such  as  copper  sulphate  is  taken 
as  a  control.  If  the  solution  undergoing  analysis  is  colored  a 
picture  similar  to  that  shown  in  Fig.  2  may  be  seen  after  a  day 
or  two.  While  non-colloids,  that  is  molecular-disperse  or  true 
solutions,  gradually  spread  down  into  the  jelly,  colloid  solutions 
do  this  only  very  slightly  or  not  at  all.  In  other  words,  substances 
in  the  colloid  state  practically  do  not  diffuse  at  all.  At  the  best  they 
diffuse  with  extreme  slowness  when  compared  with  the  behavior  of 
substances  in  molecular  solution. 

If  it  is  feared  that  a  liquid 'of  high  specific  gravity  may  by 
mechanical  means  force  itself  into  the  jelly,  a  small  tube  half 
filled  with  gelatine  or  agar  may  be  placed  mouth  downward  into 
the  solution  contained  in  a  second  larger  vessel.  The  tube  is 
removed  after  a  few  days  and  carefully  washed  when  it  also  will 
show  the  phenomena  that  have  been  described.  If  the  liquid 
under  examination  is  light  colored  or  colorless  the  test  tube  con- 
taining the  gelatine  or  agar  is  dipped  for  an  instant  into  hot 
water  so  that  the  jelly  slips  out.  This  is  then  divided  into  several 
slices  of  equal  size,  and  the  individual  slices  are  examined  analyt- 
ically for  their  content  of  the  substance  in  question. 

This  method  is  not  generally  applicable  to  the  analysis  of 
isocolloids,  nor  when  marked  chemical  or  colloid-chemical  reac- 
tions take  place  between  the  jelly  and  the  liquid  under  examina- 
tion. Under  such  circumstances  it  is  necessary  to  resort  to 
other  methods. 

10.  Dialysis  Experiments. — (b)  Dialysis,  a  process  closely  re- 
lated to  diffusion,  depends  upon  the  fact  that  animal,  plant,  and 
artificial  membranes  hold  back  substances  in  colloid  solution 
while  they  allow  substances  in  molecular  solution  to  pass  through 
them  whenever  such  a  membrane  separates  the  liquid  under 
examination  from  the  pure  dispersion  means  (the  solvent) .  Parch- 
ment bags,  so-called  diffusion  sacs  made  in  one  piece  (see  Figs. 
3  and  4),  pig  and  fish  bladders,  and  artificially  prepared  colloid 
membranes  form  the  most  convenient  as  well  as  the  most  fre- 
quently employed  of  these.  The  last-named  are  made  by  stick- 
ing a  large,  well-cleansed  test  tube  into  collodion  dissolved  in 


PRACTICAL   INTRODUCTION 


II 


ether  and  alcohol,  permitting  the  collodion  layer  formed  to 
harden  slightly  by  evaporation,  repeating  the  process  if  necessary, 
and  then  hardening  the  whole  by  washing  in  water.  The  collo- 
dion bag  is  then  carefully  drawn  off  the  tube.1  When  only  small 
amounts  of  liquid  are  to  be  analyzed  colloidally,  diffusion  sacs 
(Schleicher  and  Schiill)  arranged  as  shown  in  Fig.  3  are  par- 
ticularly useful.  For  this  purpose  a  small  Erlenmeyer  flask  is 
used,  into  the  neck  of  which  the  diffusion  sac  fits  snugly;  the  flask 
is  first  filled  with  the  pure  solvent  while  the  liquid  under  examina- 
tion is  poured  into  the  sac  which  is  then  closed  with  a  cork  stopper. 
In  this  way,  aided  by  the  slight  swelling  of  the  sac  which  usually 
occurs,  evaporation  and  the  entrance  of  dust  into  the  liquid  are 


FIGS.  3  and  4. — Simple  arrangement  for  dialytic  analysis. 

largely  prevented.  It  is  evident  that  if  the  solutions  under  ex- 
amination are  alcoholic  or  ethereal  in  character,  collodion  sacs 
cannot  be  used.  When  dealing  with  such  volatile  liquids  it  is 
advisable  to  employ  glass-stoppered  vessels  in  which  the  dialyzer 
is  placed  or  suspended  as  shown  in  Fig.  4.  The  dialyzer  dis- 
tinguishes colloid  from  crystalloid  solutions  in  that  it  does  not  allow 
the  former  to  pass  through  the  membrane  into  the  outer  liquid.  Oc- 

1  Details  of  various  methods  of  preparation  may  be  found  in  A.  Cotton  and  H. 
Mouton:  Les  Ultramicroscopes,  117,  Paris,  1906;  L.  Bigelow,  Journ.  Am.  Chem. 
Soc.,  29,  1576  (1907);  J.  Duclaux,  Journ.  Chem.  Phys.,  7,  430  (1909);  W.  Biltzand 
A.  von  Vegesack,  Zeitschr.  f.  physik.  Chem.,  63,  369  (1909). 


1 2  COLLOID-CHEMISTRY 

casionally  we  find  that  a  colloid  "phase"  will  pass  with  a  molec- 
ularly  dissolved  phase  into  the  outer  liquid.  But  this  happens 
only  at  first.  After  the  outer  liquid  has  been  renewed  once  or 
twice,  no  more  of  the  colloid  phase  comes  through.  Sometimes 
a  dissolved  substance  will  penetrate  a  collodion  sac  when  it  is 
held  back  by  the  less  porous  parchment  paper.  In  such  cases  we 
are  evidently  dealing  with  a  " highly  disperse"  (finely  divided) 
colloid,  or  to  put  it  in  another  way,  with  a  substance  occupying  a 
position  midway  between  the  colloid  and  molecular-disperse  state. 
So-called  ultrafilters  are  used  for  more  exact  determinations  of 
the  degree  of  subdivision,  but  they  cannot  be  discussed  here  be- 
cause they  are  rather  complex  (see  later) . 

11.  Transition  Systems.- — It  will  nearly  always  be  possible  to 
determine  by  one  or  more  of  the  methods  described  whether  a 
substance  in  solution  is  in  the  colloid  or  in  the  molecular-dis- 
perse state.     At  the  same  time  it  must  be  admitted  that  we 
encounter  cases  in  which  one  and  the  same  liquid  yields  different 
results  with  different  methods.     Thus  a  pure  congo  red  shows  only 
a  faint  Tyndall  cone,^  yet  it  scarcely  diffuses  through  parchment 
paper.     Protein  solutions  behave  in   a  similar  way  in  certain 
concentrations,  etc.     For  a  complete  analysis  it  is  therefore  not 
only  advisable  but  necessary  to  employ  several  methods.     But 
even  then  it  may  occasionally  be  doubtful  whether  we  are  dealing 
with  a  colloid  or  with  a  molecular-disperse  solution.     These  cases 
constitute  the  extremely  interesting  transitional  types  between 
the  two  kinds  of  solution.     Their  state  can  be  completely  analyzed 
only  by  application  to  them  of  the  more  refined  methods  of 
colloid  and  physical  chemistry — ultramicroscopy,  ultrafiltration, 
molecular  weight  determination,  etc. 

H.  ELEMENTARY  SPECIAL  COLLOID  ANALYSIS 

12.  Suspensoids    and    Emulsoids.— When    one    undertakes 
detailed  work  with  substances  in  the  colloid  state  one  soon  dis- 
covers that  the  individual  illustrations   arrange  themselves  in 
two  classes  of  systems  which  differ  markedly  from  each  other,  in 
spite  of  the  fact  that  all  are  possessed  of  the  same  general  prop- 
erties that  we  have    already  discussed.     These  two   groups   of 
colloid  solutions  are  the  suspension  colloids   (suspensoids)   and 


PRACTICAL   INTRODUCTION  13 

the  emulsion  colloids  (emulsoids),  or  as  they  are  also  called, 
the  lyophobic  (hydrophobic)  and  lyophilic  (hydrophilic)  colloids. 
The  theoretical  basis  for  such  nomenclature  will  be  discussed 
later.  In  passing,  it  should  be  noted  that  the  two  terminologies 
are  not  entirely  synonymous,  though  for  practical  purposes  they 
may  be  so  regarded.  When  by  the  general  methods  previously 
discussed  we  have  discovered  that  we  are  dealing  with  a  colloid 
solution  we  need  next  to  determine  whether  it  is  a  suspensoid 
or  an  emulsoid.  Of  the  many  means  of  doing  this  we  describe 
the  following  because  they  are  particularly  characteristic  and 
simplest  in  character. 

13.  Viscosity. — The  viscosity  of  a  suspension  colloid,   par- 
ticularly in  low  concentration,  is  imperceptibly  greater  than  that 
of  the  pure  dispersion  means  (the  pure  solvent).     In  contra- 
distinction, the  viscosity  of  an  emulsion  colloid  even  in  low  con- 
centration is  much  greater  than  that  of  its  dispersion  means; 
in  fact  at  higher  concentrations  this  becomes  so  great  that  the 
colloid  solution  assumes  an  oily  or  even  a  gelatinous  consistency. 
Further,  the  viscosity  of  an  emulsion  colloid  generally  increases 
rapidly  with    decrease   in    temperature    which  is  not   the  case 
with   a    suspension   colloid.     The   simplest   way   of    estimating 
experimentally  the  viscosity  of  a  colloid  solution  and  its  varia- 
tions with  temperature  and  concentration  is   to   measure   the 
time  of  outflow  of  a  constant  volume  of  liquid  from  a  standard 
volumetric  (10  cc.)  pipette.     Roughly,  the  viscosity  is  inversely 
proportional  to  the  time  of  outflow. 

14.  Coagulation. — It  is  characteristic  of  colloid  solutions  that 
the  substance  in  colloid  solution  may  be  easily  precipitated  or 

coagulated' '  through  various  agencies  (see  Figs .  5  and  6) .  Electro- 
lytes such  as  neutral  salts  are  particularly  effective.  The 
suspension  colloids  are  easily  coagulated  when  minute  quantities 
of  salts,  especially  those  having  a  polyvalent  ion,  are  added  to  them, 
while  the  emulsion  colloids  are  precipitated  only  after  the  addition 
of  much  larger  quantities  of  salt.  This  is  particularly  true  of 
hydrosols,  that  is  of  colloids  having  water  as  the  dispersion 
means.  If,  for  example,  aluminium  sulphate  (ordinary  alum 
serves  the  same  purpose)  is  selected  as  the  coagulant,  it  is  found 
that  almost  all  suspension  colloids  are  precipitated  by  this  as 
soon  as  it  is  present  in  a  i  per  cent,  concentration.  Much 


COLLOID-CHEMISTRY 


higher  concentrations  are  necessary  to  precipitate  the  typical 
emulsion  colloids.  In  fact  the  coagulation  of  many  emulsion 
colloids  is  not  brought  about  until  the  neutral  salts  have  been 
added  to  the  point  of  saturation.  In  making  these  qualitative 
analyses  one  must  not  use  salts  of  the  heavy  metals,  for  they 
frequently  produce  entirely  abnormal  coagulation  effects. 

15.  Influence  of  Concentration. — One  will  occasionally  en- 
counter instances  in  which  neither  viscosity  nor  coagulation  deter- 
minations will  serve  to  distinguish  clearly  a  suspension  colloid 
from  an  emulsion  colloid.  It  is  then  advisable  to  compare  with 
each  other  rather  dilute  solutions  of  suspension  colloids  and 


FIG.  5. — Non-coagulated. 


FIG.  6. — Coagulated   through  addition 

of  2  per  cent,  sulphuric  acid. 
Coagulation  of  an  aqueous  suspension  of  lamp-black.     (After  E.  E.  Free.) 

rather  concentrated  solutions  of  emulsion  colloids.  We  encounter 
here  also  a  series  of  interesting  transitional  types  which  can  be 
accurately  analyzed  only  through  quantitative  study.  The 
suspensoid  or  emulsoid  state  is  not  a  constant  or  integral  property 
of  a  chemical  substance,  it  is  the  result  of  a  series  of  physico- 
chemical  variables  which  bring  about  a  particular  state  in  a 
chemical  substance. 

16.  The  Electric  Properties  of  Colloids. — Colloid  solutions 
have  a  characteristic  electric  behavior  which  explains  many  of 


PRACTICAL  INTRODUCTION  15 

their  peculiar  properties.  Most  substances  in  colloid  solution 
assume  an  electric  charge  toward  their  dispersion  means,  though 
the  magnitude  of  this  charge  varies  greatly.  We  are  able  to 
distinguish  between  negatively  and  positively  charged  sub- 
stances in  colloid  solutions.  The  simplest  method  of  determining 
with  which  of  these  we  are  dealing  in  a  given  case  is  to  make  use 
of  their  difference  in  behavior  (as  noted  by  F.  Fichter  and  N. 
Sahlbohm)  when  they  are  subjected  to  capillary  analysis  by  means 


FIG.  7. — Ascent  of  positively  and  negatively  charged  colloids.     (According  to 

N.  Sahlbom.) 

The  dispersion  means  ascends  the  paper  in  all  these  experiments,  but  as  shown  in 
the  left  half  of  the  photograph,  the  positively  charged  colloids  (metallic  oxides)  are 
precipitated  at  once  at  the  margin  of  immersion.  The  negatively  charged  colloids, 
on  the  other  hand,  (gold,  silver,  arsenic  sulphide,  antimony  sulphide,  Berlin  blue, 
selenium)  ascend  with  the  dispersion  means,  being  separated  from  its  upper  margin 
by  a  diffusely  stained  area. 

of  filter  paper.  If  the  lower  end  of  a  strip  of  filter  paper  is  im- 
mersed in  a  colloid  solution  one  of  two  things  may  happen  de- 
pending upon  the  character  of  the  electric  charge  of  the  colloid; 
if  the  colloid  carries  a  negative  charge  it  wanders  up  the  strip  of 
paper  along  with  its  dispersion  means.  The  colloid  may  rise  to 
a  height  of  20  centimetres  or  more  depending  upon  the  kind  of 
paper  used  and  the  special  properties  of  the  colloid.  If  the 


1 6  COLLOID-CHEMISTRY 

colloid  carries  a  positive  charge  the  dispersion  means  continues 
to  rise  to  the  normal  height,  but  the  colloid  phase  does  not.  It 
rises  to  a  point  but  slightly  above  the  level  of  the  liquid  in  which 
the  filter  paper  is  immersed,  becomes  highly  concentrated  here 
and  finally  coagulates.  Positively  charged  colloids  may  there- 
fore be  separated  from  their  dispersion  means  through  the  cap- 
illary action  of  strips  of  filter  paper.  The  behavior  is  illustrated 
in  the  accompanying  Fig.  7,  taken  from  N.  Sahlbohm. 

17.  The  Mutual  Precipitation  of  Colloids. — Another  means  of 
determining  quickly  the  character  of  the  charge  of  a  substance  in 
colloid  solution  depends  upon  the  fact  that  oppositely  charged 
colloids  precipitate  each  other.     If  two  typical  test  solutions  are 
kept  in  stock  (for  example,  a  positive  colloid  such  as  ferric  hy- 
droxide, and  a  negative  colloid  such  as  sulphur  or  arsenious  sul- 
phide)   the   charge  of   an  unknown  colloid   may   frequently  be 
determined  by  ascertaining  with  which  of  the  two  solutions  it  yields 
a  precipitate.     The  charge  of  the  unknown  colloid  is  then  the 
opposite  of  that  of  the  precipitating  colloid  the  charge  of  which  is 
known.     This   method  is  not,   however,   universally   applicable 
(see  the  section  on  coagulation  of  colloids). 

18.  Electrophoresis. — The   character   of   the   charge   of   the 
colloid  phase  may  be  determined  by  noting  the  direction  in  which 
it  moves  when  subjected  to  the  action  of  an  electric  current 
(migration  in  an  electric  field) .     To  do  this  the  colloid  is  poured 
into  a  U  tube  closed  with  corks  and  provided  with  platinum 
electrodes  which  dip  into  the  solution.     When  a  stronger  current 
is  not  available,  that  from  a  few  storage  cells  will  frequently 
suffice  to  produce  a  movement  of  the  colloid  toward  one  or  the 
other  pole  if  only  sufficient  time  be  allowed.     If  the  colloid  wanders 
toward  the  anode  it  is  negatively  charged;  if  it  wanders  toward 
the  cathode  it  is  positively  charged.     Disturbing  secondary  effects 
often  enter  into  the  behavior  of  a  colloid  when  subjected  to  the 
electric  current,  and  so  it  is  advisable  to  employ  along  with  it  the 
methods  of  colloid  analysis  already  described. 

If  the  colloid  under  examination  is  colorless  it  may  be  necessary 
to  call  in  the  aid  of  simple  analytical  methods. 

19.  Summary. — The  following  is  an  outline  of  the  methods  of 
qualitative  colloid-chemical  analysis  discussed  above. 


PRACTICAL  INTRODUCTION 


A.  ELEMENTARY  GENERAL  COLLOID  ANALYSIS 


I.  Chemically  Homogeneous  Liquids. 

(a)  Always  hylotropically  transformable,  definite  boiling 
and  freezing  points,  normal  molecular  surface  energy, 
etc. 

(&)  Physical  mixtures  of  substances  having  the  same  chem- 
ical composition,  but  different  "body  properties"  as 
different  boiling  points,  molecular  surface  energies, 
etc.;  mixtures  of  isomeric  and  polymeric  substances, 
highly  associated  liquids,  etc. 
II.  Chemically  Heterogeneous  Liquids. 

(c)  Microscopically  heterogeneous,  components  separable 
by  ordinary  filtration,  may  be  easily  sedimented  espe- 
cially if    centrifuged,  precipitate    spontaneously,  can 
usually  be  easily  resuspended  on  shaking. 

(d)  Optically  homogeneous,  at  least  in  low  concentrations, 
diffuse  well  (as  into  gelatine  or  agar-agar)  and  pass 
through  membranes  (as  when  subjected  to  dialysis),  etc. 

(e)  Often   turbid  macroscopically,   Tyndall  test  positive 
(apply  Nicol  prism  to  differentiate  from  fluorescence), 
non-diffusible  or  only  slightly  so,  will  not  pass  through 
a  membrane,  etc. 


Normal  liquids. 


Isodispersoids, 

possibly 
Isocolloids.15 


Coarse  dispersions 
(suspensions  and 
emulsions) . 

Molecujar-dis- 
perse  solutions. 


Colloid  solutions. 


B.  ELEMENTARY  SPECIAL  COLLOID  ANALYSIS 


(/)  Viscosity  not  perceptibly  greater  than  that  of  dis- 
persion means;  easily  coagulable  by  electrolytes,  espe- 
cially by  salts  with  polyvalent  ions  (i  per  cent.  alum). 

(g)  Viscosity  substantially  greater  than  that  of  pure  dis- 
persion means  even  in  low  concentrations.  Viscosity 
increases  with  lowering  of  temperature;  difficultly  coagu- 
lable by  salts  which  at  times  need  to  be  added  to 
saturation  point  to  accomplish  flocculation. 

(K)  Electric  behavior: 

1.  Colloids   can   be   separated   from   their    dispersion 
means  by  capillary  analysis  (filter-paper  experiment); 
are  precipitated  on  adding  colloid  solutions  of  sul- 
phur or  arsenious  sulphide;  wander  toward  cathode. 

2.  Can  be  separated  but  slightly,  if  at  all,  from  their  dis- 
persion means  (filter-paper  experiment);  are  precipi- 
tated by  colloid  hydroxide  solutions;  wander  toward 
anode. 

15  For  details  see  §18,  page  102. 


Suspension    col- 
loids (lyophobic 
colloids). 

Emulsion  colloids 
(lyophilic    col- 
loids). 


Positive  colloids. 


Negative  anodes. 


PARTI 

GENERAL  COLLOID-CHEMISTRY 

(THEORY  OF  THE  COLLOID  STATE) 


CHAPTER  I 

THE  GENERAL  CONSTITUTION  OF  COLLOID  SYSTEMS 
§2.  The  Colloids  as  Heterogeneous   Systems 

i.  The  Concept  of  Heterogeneity. — The  typical  colloids,  more 
particularly  the  colloid  solutions,  belong  to  the  group  of  systems 
designated  as  poly  phasic  or  heterogeneous  by  physical  chemists. 
This  is  the  broadest  as  well  as  the  best  established  generalization 
thus  far  derived  from  the  study  of  colloid-chemistry. 

By  a  phase  is  meant  any  homogeneous  part  of  a  system  differ- 
ent from  other  parts  of  the  system  and  separated  from  these  by 
abrupt  transitions.  Thus  we  distinguish  a  gaseous  and  a  liquid 
phase  in  a  closed  vessel  which  is  half  filled  with  water ;  we  obtain 
two  liquid  phases  when  we  mix  water  with  carbon  disulphide; 
we  get  a  solid  and  a  liquid  phase  when  we  shake  up  quartz  dust 
in  water.  Sudden  changes,  more  particularly  in  physical  proper- 
ties, are  encountered  as  a  rule  as  we  pass  from  one  phase  to 
another  in  these  systems,  or,  to  put  it  more  simply,  we  recognize 
that  there  exist  boundaries  or  surfaces  between  them.  These 
changes  may  be  due  either  to  the  fact  that  the  different  phases 
possess  totally  different  properties,  or  there  may  be  simply  a 
quantitative  difference  in  the  properties  possessed  by  each.  We 
may  therefore  speak  of  different  kinds  and  of  different  grades  of 
heterogeneity.  Thus  two  phases  may  be  heterogeneous  optically 
but  homogeneous  electrically  or  thermally.  It  has  further  been 
found  that,  as  a  rule,  more  of  the  physical  and  physico-chemical 
properties  of  the  different  constituents  of  a  system  change  at  the 
planes  of  contact  when  a  solid  phase  and  a  liquid  phase  coexist 
than  when  two  (non-miscible)  liquid  phases  coexist.  Failure  to 
state  the  exact  properties  to  which  they  referred  in  arguing  for 
the  heterogeneity  of  colloid  systems  has  undoubtedly  been  the 
cause  of  much  misunderstanding  in  the  discussion  of  this  question. 
It  must  also  be  remembered  that  the  general  constitution  of 
boundary  planes  must  tend  to  vary  the  more,  the  greater  the 

21 


22  GENERAL  COLLOID-CHEMISTRY 

differences  in  the  nature  and  the  larger  the  number  of  the  prop- 
erties which  change  abruptly  at  them.  In  this  sense  the  surface 
where  a  liquid  comes  in  contact  with  another  liquid  may  be  said  to 
be  less  well  denned  than  that  where  a  liquid  comes  in  contact 
with  a  solid.  The  importance  of  making  such  a  distinction  has 
repeatedly  shown  itself  in  the  literature  of  colloid-chemistry  as  we 
shall  see  later. 

2.  Physical  and  Chemical  Heterogeneity. — The  relation  be- 
tween chemical  (analytical)  and  physical  heterogeneity  is  of 
especial  importance  for  the  exact  characterization  of  the  colloids 
as  'heterogeneous  systems.  It  has  been  found  that  two  spatially 
different  parts  of  a  system  having  the  same  chemical  composition 
(chemically  homogeneous  therefore),  may  physically  show  typical 
boundaries  between  them,  in  other  words  appear  as  different  phases. 
In  illustration  of  this  fact  we  need  but  call  to  mind  the  allotropic 
modifications  of  the  elements  which  may  coexist  for  a  long  time, 
as  in  the  case  of  sulphur.  But  compounds  of  the  same  composi- 
tion, particularly  isomers  and  polymers,  may  also  form  hetero- 
geneous systems.  Thus  the  solid  particles  of  metastyrol  may  be 
suspended  in  liquid  styrol;  rubber  may  be  " dissolved"  in  iso- 
prene,  etc.  It  is  even  possible  to  have  hylotropic  phases  (different 
states)  of  one  and  the  same  chemical  substance  coexist,  though 
usually  but  temporarily.  •  Thus  water  and  steam,  or  water  and 
ice  may  coexist. 

It  follows  from  all  this  that  the  spatial  or  other  heterogeneity 
of  the  colloids  is  not  connected  with  a  chemical  heterogeneity  if 
by  the  latter  is  meant  a  difference  in  analytical  composition.  The 
existence  of  physical  boundary  planes  in  them  can  alone  be  regarded 
as  characteristic  of  them.  As  a  matter  of  fact  we  can  distinguish 
between  the  different  phases  having  the  same  chemical  composi- 
tion in  the  illustrations  instanced  above  not  only  by  differences 
in  their  chemical  reactivity,  but  by  optical  differences,  by  differ- 
ences in  states  of  aggregation,  etc.,  in  other  words  by  their  different 
"body  properties." 

But  whenever  we  speak  of  (physically)  heterogeneous  systems  we 
mean  spatial  combinations  of  coexisting  phases.  There  are  several 
types  of  these.  Thus  we  have  systems  in  which  all  the  phases 
are  liquid,  others  in  which  they  are  all  solid,  still  others  in  which 
solid,  liquid  and  gaseous  phases  are  mixed.  Heterogeneous  sys- 


GENERAL   CONSTITUTION   OF   COLLOID    SYSTEMS  23 

terns  composed  of  gases  only  cannot  long  exist,  for  gases  are 
miscible  with  each  other  in  all  proportions  (see  p.  43). 

§3.  Colloids  as  Disperse  Heterogeneous  Systems 

Ordinarily  the  number  of  phases  and  their  state  of  aggregation 
in  a  heterogeneous  system  can  be  determined  by  mere  macroscopic 
examination.  We  need  but  to  call  to  mind  the  above-mentioned 
examples  of  heterogeneous  systems.  The  following  two  pecu- 
liarities serve  to  distinguish  the  colloid  solutions  from  these 
macroheterogeneous  systems. 

1.  The  Phases  are  in  Contact  with  Each  Other  under  Condi- 
tions which  Permit  of  the  Development  of  much  Surface  between 
Them. — First,  the  amount  of  the  absolute  surface  with  which  the 
various  phases  are  in  contact  with  each  other  is  very  great.     Second, 
and  more  important  and  characteristic  is  the  fact  that  the  magni- 
tude of  the  specific  surface,1  that  is  the  quotient  of  the  surface 
divided  by  the  volume  is  extraordinarily  great  in  colloid  systems. 
This  property  may  also  be  expressed  by  saying  that  there  is  a 
great  concentration  of  surface  in  the  unit  volume.     At  first  sight 
one  might  believe  that  the  small  size  of  the  individual  particles 
characterizes  this  class  of  heterogeneous  systems.     In  fact  we 
frequently  speak  of  the  great  dispersion  or  subdivision  of  the 
phases.     Nevertheless,  it  should  be  noted  that  it  is  only  because  a 
great  number  of  such  small  particles  exist  in  a  relatively  small 
volume   that  the  peculiarities   of   such  systems   are  produced. 
From  this  it  should  also  be  clear  that  it  is  not  the  absolute  amount 
of  surface  which  is  of  such  importance  as  the  relative  or  specific 
surface. 

2.  The  Phases  are  so  Distributed  within  the  System  that 
Externally  the  Whole  Appears  Homogeneous. — Unless  an  ex- 
ceedingly small  amount  is  studied,  every  fraction  of  a  colloid 
solution  has  the  same  average  composition.     This  second  pecu- 
liarity is  evidently  closely  connected  with  that  given  above,  for  a 
spatially  uniform  distribution  of  the  phases  is  made  possible  only 
through  the  existence  of  a  great  specific  surface.     To  express  it  in 
more  exact  terms,  the  uniformity  of  this  distribution  increases 
with  the  degree  of  subdivision  of  the  phases. 

1  Wo.  Ostwald,  Pfluger's  Arch.  f.  d.  ges.  Physiol.,  94,  251  (1903).  See  also  J.  M. 
van  Bemmelen,  Die  Absorption,  Ges.  Abhandl.,  23,  Dresden,  1910. 


24  GENERAL  COLLOID-CHEMISTRY 

It  should  be  emphasized  that  the  first-mentioned  property  is 
more  important  and  more  characteristic  of  these  bodies  than  the 
uniform  distribution  of  the  phases  in  space.  It  must  be  admitted 
that  the  conception  of  "colloid"  or,  more  correctly  speaking,  of 
"colloid  state"  was  developed  in  connection  with  the  study  of 
bodies  belonging  to  the  spatially  homogeneous  systems,  that  is  to 
say,  in  connection  with  the  study  of  colloid  solutions.  But  there 
are  systems  which  also  belong  to  the  field  of  colloid-chemistry  in 
which  the  phases  are  not  uniformly  distributed  within  each  other. 
To  the  former  systems  belong,  for  example,  bodies  which  are  in 
the  course  of  swelling  or  bodies  which  have  attained  their  maxi- 
mum of  swelling  in  the  presence  of  an  excess  of  the  solution  in 
which  they  are  swelling.  On  the  other  hand,  colloid  solutions  in 
the  process  of  coagulating  are  systems  which  are  losing  their 
condition  of  spatial  homogeneity.  It  is  evident  that  processes  of 
swelling  tend  toward  the  production  of  a  spatially  homogeneous 
system  in  the  sense  in  which  this  term  was  used  above.  On  the 
other  hand,  the  process  of  coagulation  arises  in  a  homogeneous 
system.  It  is  because  these  bodies  have  in  their  past  been  in,  or 
tend  in  their  future  to  assume,  a  spatially  homogeneous  condition 
that  they  belong  to  the  class  of  systems  here  under  discussion. 

It  follows  further  from  these  considerations  that  the  colloid 
solutions  occupy  an  intermediate  position  in  colloid-chemistry, 
and  should  always  be  meant  when  colloids  in  general  are  under 
discussion.  Thomas  Graham  very  expediently  introduced  the 
special  name  of  sol  for  these  spatially  homogeneous  systems.  In 
contradistinction,  systems  in  the  course  of  passing  to  or  from  a 
condition  of  spatial  homogeneity  he  calls  gels. 

Heterogeneous  systems  with  the  two  peculiarities  mentioned 
are  known  as  disperse  heterogeneous  systems,  as  suggested  by 
Wolfgang  Ostwald1  or  simply  as  "dispersoids,"  an  abbreviation 
proposed  by  P .  P .  von  Weimarn . 2  The  term ' '  microheterogeneous ' ' 
systems,  put  forward  by  G.  Bredig,3  is  synonymous  with  these. 
The  broadest  generalization  resulting  from  the  investigation  of 
the  colloids  seems  therefore  to  be  the  establishment  of  the  fact 
that  they  are  a  part  of  the  general  class  of  the  dispersoids,  in 

*Wo.  Ostwald,  Koll.-Zeitschr.,  I,  291,  331  (1907). 

2  P.  P.  von  Weimarn,  Koll.-Zeitschr.,  3,  26  (1908).  von  Weimarn  uses  this  term 
in  a  somewhat  modified  sense. 

3G.  Bredig,  Zeitschr.  f.  Elektrochem.,  12,  589  (1906). 


GENERAL   CONSTITUTION   OF   COLLOID    SYSTEMS  25 

other  words  the  colloid  state  of  a  substance  is  a  special  case  of 
the  dispersoid  state. 

§4.  The  Disperse  Phase  and  the  Dispersion  Means 

Detailed  examination  of  a  simple  dispersoid,  a  suspension 
let  us  say,  at  once  lets  us  recognize  the  fact  that  the  two  phases 
are  geometrically  or  structurally  different  from  each  other.  Most 
frequently  one  phase  is  composed  of  a  great  number  of  individual 
particles  which  are  separated  from  each  other.  Because  these 
particles  have  for  the  most  part  the  same  properties,  we  are 
accustomed  to  consider  them  as  a  whole  and  so  to  designate  them 
as  one  phase.  The  individual  particles  of  such  a  phase  may 
approximate  the  spherical  in  form,  but  they  may  also  be  crystal- 
line; further,  they  may  be  mobile  or  immobile.  The  second  phase 
is  usually  continuous  and  lies  between  the  particles,  droplets  or 
bubbles  of  the  first  phase.  The  first  phase  is  therefore  generally 
suspended  in  the  second.  As  we  must  frequently  distinguish 
between  the  two  phases  they  have  received  special  names.  The 
finely  subdivided  discontinuous  phase  is  called  the  disperse  phase; 
the  other  continuous  or  "  closed"  phase  is  known  as  the  dispersion 
means.1  English  writers  are  in  the  habit  of  calling  the  disperse 
phase  the  "internal  phase,"  the  dispersion  means  the  " external 
phase."  French  investigators  distinguish  between  the  "micelles, 
granules  colloidaux"  and  the  "milieu  exterieur." 

This  normal  geometrical* constitution  may  in  some  instances 
give  place  to  a  more  complex  one  depending  particularly  upon 
the  behavior  of  the  disperse  phase.  The  disperse  phase  may 
also  be  continuous  and  may  then  extend  through  the  dispersion 
means  in  the  form  of  a  reticulum  or  network.  Such  systems 
are  formed  in  the  first  stages  of  many  processes  of  coagulation. 
Evidently  when  the  disperse  phase  and  the  dispersion  means  bear 
such  a  relationship  to  each  other  the  distinction  between  them 
disappears;  one  can  at  best  only  designate  the  phase  present  in 
excess  the  dispersion  means. 

Relations  become  more  complex  when  heterogeneous  systems 
with  more  than  two  phases  are  considered.  In  the  more  im- 
portant and  typical  cases  several  disperse  phases  may  exist  in 

1  Wo.  Ostwald,  Koll.-Zeitschr.,  i,  291,  331  (1907). 


26  GENERAL  COLLOID-CHEMISTRY 

the  form  of  spatially  discrete  particles  in  a  common  dispersion 
means.  All  reactions  between  colloids,  like  those  of  the  immune 
bodies,  take  place  in  a  common  dispersion  means.  On  the  other 
hand,  systems  in  which  one  of  the  disperse  phases  is  continuous 
while  another  consists  of  individual  particles  are  represented  by 
membranes  of  colloid  origin  through  which  a  colloid  solution  is 
being  filtered. 

§5.  Specific  Surface  in  Dispersoids;  Degree  of  Dispersion 

It  has  been  stated  that  the  chief  characteristic  of  the  dis- 
persoids  is  the  great  development  of  surface  by  their  phases  or 
the  great  value  of  their  specific  surface.  It  should  now  be  noted 
that  the  following  three  kinds  of  specific  surface  may  be  dis- 
tinguished in  a  typical  diphasic  dispersoid: 

The  absolute  surface  of  the  entire  disperse  phase 
The  total  volume  of  the  disperse  phase 

The  absolute  surface  of  the  dispersion  means 
The  total  volume  of  the  dispersion  means 

The  absolute  surface  of  an  individual  particle  of  the  disperse  phase 
The  volume  of  an  individual  particle  of  the  disperse  phase 

The  first-named  specific  surface  is  undoubtedly  the  most 
characteristic  of  such  a  system.  All  three  specific  surfaces  may, 
but  two  of  them  must,  change  in  value  whenever  changes  occur  in  a 
dispersoid,  which  are  accompanied  by  changes  of  surface.  Thus 
if  a  suspension  of  quartz  particles  is  diluted,  only  the  two  first- 
named  specific  surfaces  change  in  value.  In  colloid  systems,  how- 
ever, the  third  specific  surface  or  the  size  of  the  disperse  particles 
often  also  varies  with  the  changes  in  concentration,  as  will  be 
shown  later.  As  a  rule  all  three  specific  surfaces  are  diminished 
in  processes  of  coagulation. 

Let  us  now  turn  to  a  specific  illustration  of  how  great  are 
the  values  of  the  absolute  and  the  specific  surfaces  in  a  dispersoid, 
and  how  quickly  these  values  increase  with  the  progressive  sub- 
division of  one  of  the  phases.  If  we  assume  that  the  given  volume 
undergoing  subdivision  has  and  maintains  cubical  dimensions 
then  Table  i  illustrates  the  increase  in  the  total  and  specific 
volumes  if  the  division  takes  place  decimally. 


GENERAL   CONSTITUTION   OF   COLLOID    SYSTEMS 


TABLE  i. — INCREASE  IN  THE  SURFACE  OF  A  CUBE  WITH  PROGRESSIVE  DECIMAL 

SUBDIVISION 


Length 

of  one  edge 

Number  of 
cubes 

Total  surface 

Specific 
surface 

i          cm. 

I 

6  square  cm. 

6 

i          mm.   = 

X  io~   cm. 

IO3 

60  square  cm. 

6.I01 

o  .  i      mm.   = 

X  io~  cm. 

I06 

600  square  cm. 

6.io2 

o.oi    mm.   = 

X  io~   cm. 

I09 

6000  square  cm. 

6.io3 

i  .0      /*         = 

X  io~  cm. 

I012 

6  square  m. 

6.io4 

O.I         /i             = 

X  io~  cm. 

I015 

60  square  m. 

6.io6 

O.OI     /i            = 

X  io~  cm. 

I018 

600  square  m. 

6.io8 

1  .  0        /i/i          = 

X  io~   cm. 

IO21 

6000  square  cm. 

6.io7 

O.I         /i/i          = 

X  io~   cm. 

10" 

6  hectares 

6.io8 

O.OI      (JLfJi           — 

X  io~9  cm. 

10" 

60  hectares 

6.io9 

O.OOI  fjtfj,          = 

X  io~10  cm. 

I030 

6  square  km. 

6.I01' 

Particles  somewhat  less  than  IQ/JJU  in  diameter  may  be  dis- 
tinguished optically  by  means  of  the  ultramicroscope  of  H. 
Siedentopf  and  R.  Zsigmondy.  A  cube  of  metallic  gold  subdivided 
up  to  the  limit  of  ultramicroscopic  visibility  would  therefore  have 
a  total  surface  of  over  600  square  meters  and  a  specific  surface  of 
6.  io6.  Even  at  this  point  we  begin  to  enter  the  sphere  of  molecular 
dimensions.  Lobry  de  Bruyn  and  Wolff1  for  instance,  calculated 
an  approximate  diameter  of  5/i/x  for  the  starch  molecule.  If  a 
cubic  centimeter  of  dry  starch  could  be  subdivided  into  its 
molecules,  that  is  if  it  could  be  " dissolved"  in  the  ordinary  sense 
of  the  word,  the  starch  would  present  a  total  surface  of  several 
thousand  square  meters  toward  the  solvent.  When  we  deal 
with  the  molecular  dimensions  of  gases  and  of  substances  in  crystal- 
loid solution,  assuming  for  their  average  molecular  diameter  the 
value  of  i.io"8,  we  obtain  values  of  several  hectares  for  i  cubic 
centimeter  of  dissolved  substance.  Thus  in  100  cc.  of  a  io  per 
cent,  sugar  solution  there  would  be  an  "internal  surface"  of  about 
50  hectares  when  the  smallest  possible  surface,  the  surface  of  a 
sphere,  is  assigned  to  the  sugar  molecule.  Finally,  if  it  is  assumed 
that  ions  and  electrons  are  also  separated  through  surfaces  from 
their  dispersion  means  (and  an  electrical  heterogeneity  and  the 
existence  of  electrical  surfaces  must  be  postulated  in  these)  the 
absolute  and  especially  the  specific  surfaces  attain  enormous 
values. 

1  Lobry  de  Bruyn  and  Wolff,  Rec.  Trav.  chim.  Pays,  Bas.,  23,  155  (1904). 


28  GENERAL  COLLOID-CHEMISTRY 

It  should  further  be  noted  that  the  increase  in  the  surface 
of  a  cube  with  progressive  subdivision  may  be  expressed  by 
the  formula: 


in  which  SO  is  the  total  surface  in  square  centimeters,  a,  the 
length  of  one  edge  in  centimeters,  and  m*,  the  number  of  cubes. 
The  original  volume  was  taken  as  equal  to  i  cc.  (H.  Mayer).1 

o 

Since  the  unit  of  specific  surface  =  >  then  if  a  is  taken  as 

iccm 

equal  to  i,  the  calculated  values  obtained  for  surface  represent  at 
the  same  time  the  specific  surface  or  the  degree  of  dispersion.2 

The  concept  of  specific  surface  may  conveniently  be  replaced 
by  the  somewhat  clearer  one  of  "degree  of  dispersion"  Thus 
we  may  say  that  the  degree  of  dispersion  increases  greatly  with 
progressive  subdivision  of  a  given  phase,  etc. 

As  is  well  known,  the  surfaces  of  solid  and  liquid  bodies  of 
even  ordinary  dimensions  already  exhibit  a  whole  series  of  peculiar 
phenomena,  the  intensity  of  which  increases  in  direct  proportion 
with  the  absolute  and  specific  surfaces  of  the  bodies.  As  examples 
might  be  mentioned  the  condensation  of  gases  on  solid  surfaces, 
the  manifold  effects  of  surface  tension  in  liquids,  the  fact  that  the 
majority  of  electrical  phenomena  appear  at  surfaces,  etc.  It 
should  be  remembered,  however,  that  in  such  behavior  the  absolute 
surface  is  less  responsible  for  these  phenomena  than  the  specific 
surface.  Thus  a  few  milligrams  of  platinum  black  have  an  effect 
upon  an  explosive  gas  mixture  which  is  not  equaled  by  that  of 
several  square  meters  of  sheet  platinum,  for  while  they  may  have 
approximately  equal  absolute  surfaces  the  former  has  an  enormously 
greater  specific  surface.  We  are  driven  to  conclude  that  all  the 
phenomena  observable  at  ordinary  surfaces  increase  enormously 
in  intensity  and  that  they  may  even  change  qualitatively  when  we 
come  to  deal  with  dispersoids  with  their  immense  internal  surfaces. 
There  are  also  certain  forms  of  energy  that  play  an  insignificant 
r61e  in  macroheterogeneous  systems,  but  which  play  an  enormous 
one  in  dispersoids.  These  are  discussed  in  detail  later. 

1  H.  Mayer,  Kolloidchem.  Beihefte,  i,  62  (1909). 

2  See  also  Wilh.  Ostwald,  Grundr.  d.  allg.  Chemie,  4  Aufl.  531,  Leipzig,  1909. 


GENERAL  CONSTITUTION   OF   COLLOID   SYSTEMS  2  9 

§6.  Classification  of  the  Dispersoids  According  to  Their  Degree 

of  Dispersion 

i.  Classification  of  Zsigmondy. — It  is  evident  that  either  the 
degree  of  dispersion  or  the  number  of  phases  in  a  system  may  be 
used  for  classifying  the  dispersoids.  The  mere  number  of  phases 
is  relatively  unimportant  as  a  means  of  classification,  for  the 
majority  of  the  dispersoids  and  of  the  colloids  in  particular  are 
either  diphasic  or  triphasic.  Classification  on  the  basis  of  the 
degree  of  subdivision  permits  of  finer  distinctions. 

R.  Zsigmondy1  has  developed  a  classification  on  this  basis. 
According  to  him  the  field  of  colloid-chemistry  occupies  a  middle 
position  among  the  dispersoids  thus  far  known.  Particles  about 
o.iju  in  diameter,  that  is,  particles  with  a  specific  surface  of  about 
6.io5  (see  Fig.  8),  are  stated  by  R.  Zsigmondy  to  represent  the 
lower  limit  of  dispersion.  The  size  of  such  particles  is  about  that 
of  the  particles  in  emulsions  and  suspensions  which  no  longer 
undergo  separation.  The  value  o.i/z  about  represents  the  limit 
of  microscopic  visibility.  According  to  Zsigmondy  the  field  of 
colloid-chemistry  begins  with  particles  of  this  size  and  extends 
up  to  particles  about  IMM  in  size,  that  is,  to  such  as  have  a  specific 
surface  or  degree  of  dispersion  of  about  6.io7,  assuming  that  the 
particles  are  cubiform.  The  value  i//ju  is  somewhat  less  than  the 
diameter  of  the  smallest  particles  hitherto  observed  by  ultramicro- 
scopic  means  (about  6/*ju).  On  this  basis  of  classification  the 
colloids  represent  dispersions  of  a  magnitude  varying  between 
6.io5  and  6.io7. 

H.  Siedentopf2  and  R.  Zsigmondy3  have  proposed  a  nomen- 
clature for  the  individual  particles  of  typical  dispersoids  which  is 
based  upon  their  degree  of  dispersion.  Particles  visible  under  the 
microscope  are  termed  "microns,"  while  those  which  can  be 
seen  only  by  the  application  of  ultramicroscopic  methods  are 
called  "submicrons"  or  "ultramicrons."  The  disperse  phase  of 
colloid  solutions  would  therefore  be  made  up  of  submicrons 
(ultramicrons).  It  can  be  shown  in  several  ways  that  particles 
exist  whose  size  we  know  to  be  beyond  that  of  ultramicro- 
scopic visibility.  They  must  therefore  be  less  than  I/AJU  in 

1  R.  Zsigmondy,  Zur  Erkenntnis  der  Kolloide,  22,  Jena,  1905. 

2  H.  Siedentopf,  Berl.  klin.  Woch.,  Nr.  32,  (1904). 

3  R.  Zsigmondy,  Zur  Erkenntnis  der  Kolloide,  87,  Jena,  1905. 


GENERAL  COLLOID-CHEMISTRY 


diameter.     These  particles  to  which  molecules  and  the  products 
of  their  dissociation  belong,  are  called  "  amicrons" 

The    accompanying   Fig.  8    (based    chiefly   on    the    data    of 
R.  Zsigmondy)  is  designed  to  illustrate  approximately  the  rela- 


Precipil-al-ed  parhcie 
of  gold,  about-  75  w 


Starch  Chloroform  Hydrogen 
snolecule  molecule  molecule 
-StJfj  abouf0.dv{j  abouKUuv 


FIG.  8. — Comparison  of  particles  of  different  size. 

The  large  circle  corresponds  to  the  diameter  of  a  human  red  blood  corpuscle 
(about  7.5  /*);  the  large  pentagon  to  that  of  a  rice  starch  granule  of  medium  size 
(about  7.0  n).  The  particles  enclosed  in  the  frame  are,  in  comparison  with  the  rest 
of  the  figure,  enlarged  333  times. 

The  figure  has  been  constructed  from  data  and  tables  given  in  R.  Zsigmondy 
(Zur  Erkenntnis  der  Kolloide,  Jena,  1905).  The  values  for  the  mastic  suspension 
are  taken  from  /.  Perrin's  studies  [Kolloidchem.  Beihefte  i,  221  (1910)]. 


tive  sizes  of  the  particles  in  typical  dispersoids  which  have  been 
the  object  of  study.  According  to  this  diagram  human  blood 
corpuscles,  starch  granules,  kaolin,  and  mastix  particles  would  be 
microns,  gold  particles  would  be  submicrons,  while  the  finest  gold 


GENERAL   CONSTITUTION  OF   COLLOID   SYSTEMS  31 

particles,  starch  molecules,  etc.,  which  cannot  be  made  out  ultra- 
microscopically  would  be  amicrons. 

It  seems  of  interest  to  give  here  the  estimated  diameters  of  a 
number  of  molecules.  The  smallest  molecule  seems  to  be  that 
of  hydrogen  gas,  0.067  to  o.i 59^^;  water  vapor  has  a  molecular 
diameter  approximating  o.i  13^^;  carbon  dioxide  one  of  about 
0.285/zju,1  etc.  Different  methods  of  calculation  yield  different 
values,  yet  all  approach  the  magnitude  O.I/ZM  or  i.io~8  cm.  The 
molecular  diameters  of  hydrated  ions  have  recently  been  measured 
in  several  ways.2  The  molecular  diameter  of  NaCl  was  found  to 
be  0.26^;  that  of  sugar,  o.7//ju,  etc. 

2.  Classification  of  Dispersoids  According  to  Their  Degree  of 
Dispersion. — It  follows  from  Zsigmondy's  classification  that 
dispersoids  having  a  very  small  or  a  very  high  degree  of  dispersion 
do  not  belong  to  the  systems  to  be  specially  considered  in  this  book. 
Such  dispersoids  should  have  special  names.  Dispersoids  with  a 
degree  of  dispersion  of  less  than  6.io5,  that  is  microscopic  sus- 
pensions, emulsions,  and  foams,  might  be  called  "true  or  coarse 
dispersions"  while  dispersoids  with  a  degree  of  dispersion  higher 
than  6.io7  might  be  termed  "molecular  dispersoids."  Roughly,  the 
molecular  dispersoids  correspond  with  Thomas  Graham's  "  crystal- 
loids." As  this  term  is  based  upon  a  property  which  need  scarcely 
determine  the  degree  of  dispersion  it  is  not  as  free  from  objec- 
tion as  that  which  I  suggest.  Since  molecules  may  dissociate  into 
smaller  particles,  into  ions,  we  obtain  systems  which  may  be 
designated  as  "ionically  disperse"  or  as  "ionic  dispersoids,"  as 
suggested  by  The  Svedberg.3  It  should  be  remembered,  how- 
ever, that  ions  are  by  no  means  always  the  products  of  dissociated 
molecules,  and  especially  is  this  not  true  if  such  appear  in  colloid 
solutions.  Such  ions  need  not  therefore  have  a  higher  degree  of 
dispersion  than  the  colloid  particles  themselves.  This  will  be 
discussed  later  in  the  section  on  the  electrochemistry  of  the 
colloids. 

It  has  further  been  found  that  the  specific   surface  of  the 

1  These  figures  are  taken  from  a  table  on  p.  64  of  the  excellent  publication  of 
W.  Mecklenburg,  Die  exper.  Grundlegung  der  Atomistik,  Jena,  1910.     The  various 
methods  of  calculation  may  also  be  found  there. 

2  See,  for  example,  the  summaries  of  G.  H.  Washburn,  Jahrb.  d.  Radioaktivitat, 
5,  493  (1908);  6,  69  (1900). 

8  The  Svedberg,  Stud.  z.  Lehre  v.  d.  koll.  Losungen.  Nov.  Act.  R.  Soc.  Scient. 
Upsaliensis,  Ser.  IV,  II,  i  (1907). 


32  GENERAL  COLLOID-CHEMISTRY 

colloids  may  vary  within  the  limits  calculated  by  Zsigmondy, 
that  is  to  say,  between  6.io5  and  6.io7.  We  may  therefore 
expect  to  find  that  colloid  solutions  contain  particles  of  different 
sizes.  Experimental  study  has  confirmed  this  expectation.  Not 
only  have  different  colloids  very  different  degrees  of  dispersion, 
but  one  and  the  same  substance  may  exist  in  different  degrees  of 
subdivision  in  a  given  dispersion  means.  As  an  example  may  be 
cited  a  series  of  carefully  studied  aqueous  gold  dispersoids  in- 
vestigated by  R.  Zsigmondy.1 

TABLE  2.  —  AQUEOUS  GOLD  DTSPERSOJDS  or  DIFFERENT  DEGREES  OF  DISPERSION 

Designation  of  the  solution*  Color  of  the  dispersoid 


Au3-a  Rose  ...................  |  About  6  .  o 

Au92  Bright  red  ..............  j  About  10  .o 

Au97  Bright  red  ..............  i  15.3 

Au92b  Bright  red  ..............  j  17.0 

Au9ia  Violet  red  ..............    About  23  .o 

Au8sa  Violet  red  ..............  32.0 

Au2  Purple  red  ..............  !  38  .  o 

Gold  suspension  a  ..........  Violet  red  ..............  45  .o 

Gold  suspension  b  ..........  Bright  red  ..............  95  .o 

Gold  suspension  c  ..........  Bluish  .................  130  .  o 

*  The  designations  are  those  of  Zsigmondy  (I.e.}. 

Zsigmondy  and  other  investigators  have  prepared  gold  dis- 
persoids in  which  the  size  of  the  particles  could  not  be  determined. 
They  must  therefore  have  been  smaller  than  6^. 

This  variability  in  the  degree  of  dispersion  within  the  limits 
characteristic  of  colloid  solutions  has  been  recognized  in  the 
literature  of  colloid-chemistry  by  distinguishing  between  sub- 
stances having  a  "strong  or  a  weak  colloidality,"  and  different 
"degrees  of  colloidality."  Substances  have  also  been  designated 
as  systems  "slightly,  intermediately,  highly,  or  completely  colloid," 
or  "coarsely  disperse,  finely  disperse,"  etc.  The  term  highly 
colloid  is  synonymous  with  highly  disperse,  etc.  It  is  also  at 
times  advisable  to  distinguish  between  super  molecularly-dis  per  sed 
phases  (as  in  the  case  of  ions)  and  submolecularly-dispersed 
phases. 

1  R.  Zsigmondy,  Zur  Erkenntnis  d.  Kolloide,  104,  Jena,  1905. 


GENERAL   CONSTITUTION   OF    COLLOID    SYSTEMS 


33 


The  following  outline  gives  graphically  a  classification  of  the 
dispersoids  according  to  their  degree  of  dispersion. 


True  or  coarse  dispersions 

(suspensions,    emulsions, 

etc.). 
Size  of  the  particles  of  the 

disperse     phase     greater 

than  o.i/t. 
Specific  surface  <  6.io5. 


DISPERSOIDS 


Colloid  solutions. 


Size  of  the  particles  of  the 
disperse  phase  between 
0.1/4  and  1/j.fj.. 

Specific  surface  between 
6.io5  and  6.io7. 

Colloidality  decreases 


Molecular    and    supermo- 

lecular  dispersoids  (solu- 

toids).1 
Size  of  the  particles  of  the 

disperse  phase  about  i/u/i 

or  less. 
Specific  surface  >  6.io7. 


Degree  of  Dispersion  increases 

P.  P.  von  Weimarn2  has  repeatedly  emphasized  that  the 
so-called  " super saturated  solutions"  (in  which  we  are  justified  in 
assuming  the  existence  of  larger  molecular  aggregates)  occupy  a 
position  between  the  colloid  and  the  molecular-disperse  systems. 
But  there  seems  to  be  no  reason  for  believing  as  von  Weimarn 
does  that  supersaturated  solutions  always  represent  transitions 
between  colloid  and  molecular-disperse  systems;  or  for  believing 
that  such  transition  types  must  appear  every  time  we  pass  from  a 
high  degree  of  dispersion  to  a  lower  one  or  vice  versa.  The  con- 
cept of  supersaturation  embodies  in  itself  no  information  regarding 
degree  of  dispersion  which  alone  is  the  criterion  for  the  type  of 
classification  here  under  consideration.  Supersaturation  consti- 

1  This  name  was  proposed  by  P.  P.  von  Weimarn,  Koll.-Zeitschr.,  7, 155  (1910). 
It  should  be  noted  that  von  Weimarn  wishes  the  terms  "colloid,"  "colloid  solution," 
etc.,  avoided  and  replaced  by  the  more  general  terms  " dispersoid,"  "dispersed  solu- 
tion," etc.,  while  the  term  "colloid -chemistry"  is  to  be  replaced  by  "dispersoid 
chemistry."    In  spite  of  the  fact  that  I  was  the  first  to  propose  the  extension  of  the 
study  of  the  colloids  to  that  of  the  disperse  systems  and  first  suggested  a  suitable 
nomenclature,  yet,  for  obvious  reasons  I  do  not  deem  it  advisable  to  eliminate  the 
use  of  the  term  "colloid."     Even   the  fact  that  the  word  "colloid"  originally 
had  a  different  meaning,  namely,  a  more  special  one  than  it  now  has,  does  not  justify 
the  proposed  measure.     The  word  "molecule,"  for  example,  has  not  disappeared 
from  science  even  though  its  exact  meaning  has  changed  frequently  and  consider- 
ably.    A  dispersoid  chemistry,  in  other  words,  a  chemistry  dealing  with  disperse 
systems  of  all  degrees  of  dispersion,  does  of  course  exist.     Nevertheless,  persistence 
in  the  use  of  the  term  colloid  for  at  least  that  portion  of  this  more  general  science  with 
which  this  work  deals  seems  to  be  justified  on  historical  and  other  grounds.     See  my 
preface  to  the  second  edition  of  this  work. 

2  See  P.  P.  von  Weimarn,  Koll.-Zeitschr.,  6,  179  (1910);  and  for  greater  details 
Kolloidchem.  Beihefte,  i,  331  (1910). 


34  .  GENERAL  COLLOID-CHEMISTRY 

tutes  a  possible  but  not  the  sole  means  of  preparing  submolecular 
dispersoids.  Such  may  be  prepared  by  "  direct  methods  of 
dispersion." 

3.  Defects  of  this  Principle  of  Classification. — The  following 
should  be  noted  regarding  the  classification  of  the  dispersoids 
according  to  their  degree  of  dispersion. 

The  degree  of  dispersion  is  manifestly  a  continuously  variable 
quantity,  and  so  it  is  self-evident  that  it  may  have  any  possible 
value  between  the  extremes  which  characterize  individual  classes 
of  dispersoids.  As  a  matter  of  fact,  transitional  values  between 
those  which  characterize  the  field  of  colloid  solutions  and  those 
which  characterize  the  molecular  dispersoids,  or  between  those  of 
the  former  and  those  of  the -coarse  dispersions  are  not  only  con- 
ceivable but  have  been  demonstrated  experimentally.  The  exist- 
ence of  transitional  values  may  be  deduced  from  Table  2,  for  at 
its  top  are  gold  dispersoids  with  particles  approaching  molecular 
values  in  size,  while  at  its  bottom  are  suspensions  which  can  be 
resolved  under  the  microscope.  An  analogous  series  of  dispersoids 
in  which  the  degree  of  dispersion  varied  between  points  lying 
beyond  either  side  of  the  field  embraced  by  the  colloid  solutions 
was,  among  others,  prepared  by  H.  Picton  and  S.  E.  Linder1 
at  an  early  period  in  the  history  of  colloid-chemistry.  Dispers- 
oids of  arsenious  trisulphide  in  water  were  used.  The  size  of  the 
particles  in  these  could  not  be  determined  directly,  but  that  their 
degree  of  dispersion  varied  greatly  was  clearly  demonstrated  by 
their  very  different  degrees  of  diffusibility. 

In  the  face  of  these  facts  it  must  be  admitted  that  the  classi- 
fication of  the  dispersoids  according  to  their  degrees  of  dispersion 
is  an  arbitrary  one.  But  while  this  is  so,  there  is  undoubtedly 
a  practical  justification  for  the  distinctions  proposed.  The 
dispersion  values  given  were  chosen  because  with  changes  in  them 
abrupt  changes  occur  in  other  properties  of  the  dispersoid  also. 
Thus  the  particles  of  a  dispersoid  with  a  diameter  of  o.i/i  are 
not  only  no  longer  visible  under  the  microscope,  but  at  this  de- 
gree of  dispersion  they  also  lose  their  diffusibility,  they  no  longer 
settle  out  spontaneously,  they  do  not  pass  through  a  dialyzing 
membrane,  they  no  longer  produce  changes  in  the  freezing  and 
boiling  points  of  their  dispersion  means,  etc.  On  the  other  hand, 
1  H.  Picton  and  S.  E.  Linder,  J.  Chem.  Soc.,  61,  148  (1892);  ibid.,  67,  63  (1895). 


GENERAL   CONSTITUTION   OF    COLLOID    SYSTEMS  35 

all  these  properties  rise  and  fall  in  value,  greatly  and  suddenly, 
when  molecular  dimensions  are  approached.  These  ^continuous 
changes  of  other  properties  therefore  form  the  true  basis  for  the 
classification  of  the  dispersoids  on  the  basis  of  their  degree  of 
dispersion.  But  that  a  quantitative  characterization  of  the 
dispersoids  according  to  their  degree  of  dispersion  is  important 
is  evidenced  by  the  fact  that  dispersion  in  itself  must  be  regarded 
as  the  chief  characteristic  of  the  substances  with  which  this  book 
deals. 

4.  Polydispersoids. — It  has  frequently  been  found  in  deter- 
mining the  degree  of  dispersion  in  dispersoids,  such  as 
colloid  solutions,  that  the  individual  particles  of  the  disperse 
phase  are  of  different  sizes,  in  other  words  the  degree  of  dispersion 
of  the  disperse  phase  must  be  described  as  multiple.  Accord- 
ing to  L.  Michaelis,1  examples  of  such  systems  are  found  in 
the  aqueous  solutions  of  certain  dyes,  such  as  fuchsin,  methyl 
violet,  etc.  In  these  there  is  a  molecular-disperse  phase  in  addi- 
tion to  a  phase'  observable  under  the  microscope  or  ultra-micro- 
scope. Many  protein  solutions  probably  behave  in  an  analogous 
way,  as  may  be  inferred  from  their  ultrafiltration  behavior  (see 
later) ;  but  even  the  individual,  ultramicroscopically  observable 
particles  in  dispersoids  (as  in  those  of  gold)  are  frequently  of 
different  sizes.  It  follows  therefore  that  in  practice  we  can 
only  speak  of  an  average  dispersion  value.  The  importance  of 
the  simultaneous  existence  of  particles  of y  different  sizes  in  one 
and  the  same  dispersion  means  in  many  questions  of  colloid- 
chemistry,  for  example  in  that  of  their  stability,  will  be  discussed 
in  detail  later. 

These  systems  in  which  the  disperse  phase  is  composed  of 
particles  having  different  degrees  of  dispersion  may  be  called 
polydisperse  systems  or  poly  dispersoids. 

5.  Dispersoids  Varying  with  Changes  in  Concentration. — 
In  a  number  of  molecular  as  well  as  colloid  dispersoids  the  re- 
markable fact  has  been  observed  that  the  degree  of  dispersion 
varies  progressively  with  changes  in  concentration.  In  all  the 
cases  thus  far  studied  it  decreases  with  increasing  concentration. 
Cane  sugar,  for  example,  in  dilute  solution  has  all  the  typical 

1L.  Michaelis,  Deutsche  medizin.  Wochenschr,  Nr.  24  (1904);  Virchow's  Arch., 
179,  195 


30  GENERAL  COLLOID-CHEMISTRY 

attributes  of  a  molecular  dispersoid.  But  when  cane  sugar  solu- 
tions of  higher  concentrations  are  investigated  by  applying  the 
Tyndall  test  to  -them  it  is  found  that  they  show  an  intense  light- 
cone  thus  proving  themselves  submolecularly  disperse.  Entirely 
analogous  observations  have  been  made  on  solutions  of  various 
salts  such  as  aluminium  sulphate,  and  on  those  of  certain  dyes, 
proteins,  etc.  No  doubt  careful  investigation  will  demonstrate  the 
wide-spread  nature  of  this  remarkable  fact.  It  should  be  added 
that  such  a  progressive  decrease  in  the  degree  of  dispersion  by 
simply  changing  the  quantitative  relations  of  the  dispersoid  to  the 
dispersion  means  may  be  demonstrated  by  yet  other  than  purety 
optical  methods.  Further  details  will  be  given  in  discussing  the 
individual  physico-chemical  properties  of  the  colloids  especially 
in  the  chapter  on  their  internal  changes  of  state.  Analogous 
phenomena  are  encountered  in  studying  the  properties  of  mo- 
lecularly  and  supermolecularly  dispersed  systems,  being  then  de- 
scribed as  "polymerizations,  condensations,"  etc. 

We  will  call  these  systems,  among  which  many  colloid  disper- 
soids  appear,  "concentration-variable  systems" 

6.  Temperature -variable  Dispersoids. — Just  as  the  degree  of 
subdivision  of  a  dispersoid  may  vary  with  changes  in  concentra- 
tion, it  may  also  vary  with  changes  in  temperature.     As  far  as  we 
know  now,  raising  the  concentration  of  a  dispersoid  produces  the 
same  type  of  change  as  lowering  its  temperature.     A  disperse 
system  therefore  tends  to  become  less  disperse  when  the    tem- 
perature is  lowered.     Such  anomalous  behavior  in  "true"  solu- 
tions has  generally  been  explained  by  saying  that  the  substances 
"polymerize"  or  "condense."1     An  analogous  behavior,  resulting 
in  diminutions  of  degree  of  dispersion,  is  found  in  even  greater 
degree  in  colloid  systems.     Here  we  can  only  point  out  the  fact; 
it  is  dealt  with  in  detail  in  the  chapters  on  internal  changes  in 
state,  more  particularly  in  that  on  gelation. 

Dispersoids  showing  this  property  are  called  "temperature- 
variable  dispersoids" 

7.  Complex  Dispersoids. — There  exists  another  class  of  com- 
plex systems  which  is  interesting  for  both  the  theory  and  the 
practice  of  colloid-chemistry.     It  is  characterized  by  the  fact  that 
each  component  of  such  systems,  both  disperse  phase  and  dispersion 

1  For  examples  and  literature  see  H.  Schade,  Koll.-Zeitschr.,  7,  26  (1910). 


GENERAL   CONSTITUTION   OF   COLLOID    SYSTEMS  37 

means,  is  in  itself  a  dispersoid.  Evidently  the  degree  of  dispersion 
in  these  individual  dispersoids  must  always  be  higher  than  that  of 
the  compound  dispersoid.  And  in  fact,  the  best-known  examples 
of  such  systems  are  those  in  which  the  individual  dispersoids  have 
a  molecular  degree  of  dispersion,  while  the  compound  dispersoid  is 
colloid  or  molecular-disperse  in  character. 

The  best  examples  of  such  "complex  dispersoids"  are  found 
among  the  emulsions,  that  is  among  those  systems  in  which 
both  phases  are  liquid.  Pretty  instances  are  formed  by  the 
so-called  critical  mixtures  of  liquids  and  their  analogues.  As  is 
well  known,  it  is  possible  at  suitable  concentrations  and  at  suitable 
temperatures  to  make  a  dispersoid  of  two  liquids  which  have  a 
limited  molecular  solubility  in  each  other.  As  an  example  may 
be  cited  the  production  of  an  emulsion  of  phenol  in  water.  Since 
all  liquids  are  mutually  soluble  to  some  extent  at  least,  this  type 
constitutes  the  bulk  of  the  dispersion  systems  having  a  liquid-liquid 
composition.  Such  complex  dispersoids  are  characterized  by  the 
fact  that  changes  in  the  concentration  or  in  the  temperature  of  the 
macrodisperse  system  are  accompanied  by  changes  in  the  composition 
of  the  microdispersoids.  Thus  when  droplets  of  phenol  are  dis- 
persed in  water,  both  phases  contain  phenol  as  well  as  water.  If 
the  concentration  of  the  emulsion  is  changed  through  the  addition 
of  one  of  its  components,  for  example  water,  the  composition  of 
both  microdisperse  phases  is  also  changed.  As  more  water 
is  added  the  phenol  phase  becomes  progressively  richer  in  water 
until  a  limit  is  reached  (until  the  phenol  is  saturated  with  water), 
etc.  Variations  in  temperature  produce  analogous  effects. 

Another  peculiarity  of  these  complex  dispersoids  which  should 
be  emphasized  is  that  in  addition  to  the  fact  that  the  composition 
of  the  individual  phases  changes  with  variations  in  concentration 
or  temperature,  their  degree  of  dispersion  does  also,  and  apparently 
whenever  a  change  is  produced  in  the  total  concentration.  Thus 
the  droplets  in  mixtures  of  liquids  of  limited  mutual  solubility 
become  progressively  smaller  as  the  mixtures  approach  the  so-called 
critical  concentration,  and  disappear  altogether  at  the  "  critical 
point;"  in  other  words,  the  droplets  become  molecular-disperse. 
But  at  constant  temperature  the  critical  concentration  is  always 
less  than  the  concentration  of  the  fluids  in  a  coarsely  disperse 
state.  Here  again  there  exists  exactly  the  same  variation  in 


3  GENERAL  COLLOID-CHEMISTRY 

degree  of  dispersion  with  change  in  concentration  that  was 
previously  described.1 

What  was  said  above  regarding  simple  dispersoids  holds  for  the 
influence  of  temperature  on  composition  and  degree  of  dispersion 
in  complex  dispersoids  also.  The  complex  dispersoids  are  con- 
centration-variable and  temperature-variable  systems. 

Dispersoids  with  a  liquid  dispersion  medium  and  a  solid 
dispersion  phase  may  also  form  complex  systems,  but  up  to  the 
present  time  these  have  been  studied  but  little.  It  is  self-evident 
that  a  solid  particle  floating  in  a  liquid  may  either  take  up  part  of 
it  into  itself,  or  attach  a  layer  of  it  to  itself.  Such  behavior  may 
be  observed  macroscopically  when  solid  gelatine  is  pulverized  and 
thrown  into  cold  water.  Each  particle  then  "  swells,"  that  is,  it 
absorbs  water,  but  if  the  temperature  is  low  enough  it  does  not 
lose  entirely  the  properties  of  a  solid.  But  the  properties  of  a 
solid,  such  as  constancy  of  form  and  elasticity,  become  less  marked 
as  the  solid  particles  take  up  more  water  or  as  the  temperature 
rises.  This  very  important  behavior,  which  therefore  consists  in 
an  approximation  of  the  previously  solid  state  to  that  of  a  liquid, 
will  be  discussed  in  detail  later  (see  page  44). 

Let  it  further  be  pointed  out  that  complex  dispersoids  may  be 
expected  to  appear  more  frequently  in  systems  composed  of  two 
liquid  phases  than  in  those  composed  of  a  liquid  and  a  solid  phase. 
This  depends  upon  the  fact  that  a  greater  mutual  molecular 
miscibility  may  be  assumed  to  exist  in  the  case  of  two  liquids  than 
in  the  case  of  a  liquid  and  a  solid  phase,  and  second,  upon  the  fact 
that  the  "solubility"  of  two  liquids  is  usually  mutual.  Both 
phases  will  therefore  be  disperse  in  a  liquid-liquid  dispersoid. 
On  the  other  hand,  while  we  may  be  able  to  speak  of  the  "solu- 
bility" of  a  solid  phase  in  a  liquid  dispersion  means,  we  will  only 
infrequently  be  able  to  speak  of  the  "solubility"  of  the  dispersion 
means  in  the  solid  disperse  phase. 

It  seems  of  interest  to  mention  in  this  connection  that  many 
reasons  have  recently  been  found  for  assuming  the  existence  of 
similar  phenomena  in  molecular  and  ionic  dispersoids.  Such 
complexes  are  called  "solvates,"  or  if  they  occur  in  aqueous 

1  It  should  be  noted  that  concentration  is  always  regarded  as  the  quotient  of  the 

amounts  of  the  ;.  In  the  range  above  the  critical  point  this  fraction 

dispersion  means 

is  reversed,  in  that  the  dispersion  means  becomes  the  disperse  phase  and  vice  versa. 


GENERAL   CONSTITUTION   OF   COLLOID    SYSTEMS  39 

solutions,  " hydrates."  In  these  compound  disperse  phases  there 
also  occur  variations  in  composition  with  changes  in  concen- 
tration or  in  temperature  entirely  similar  to  those  discussed 
above. 

8.  Transition  Phenomena. — The  transition  phenomena  ob- 
served in  passing  from  the  members  of  one  class  of  dispersoids  to 
those  of  another  having  a  different  degree  of  dispersion  are  par- 
ticularly interesting.  Our  knowledge  of  the  properties  of  dis- 
persoid  systems  is  at  present  distributed  in  such  a  way  that  we 
may  say  we  know  a  great  deal  about  typical  molecular  disper- 
soids, somewhat  less  about  typical  colloids,  and  still  less  about 
typical  coarse  dispersions.  But  the  atypical  representatives  of 
all  three  classes,  that  is,  the  transition  forms  between  coarse  dis- 
persions and  colloids  on  the  one  hand,  and  between  colloids  and 
molecular  dispersoids  on  the  other,  have  been  almost  entirely 
neglected.  There  is  an  historical  reason  for  this  state  of  affairs. 
As  is  well  known,  the  founder  of  colloid-chemistry,  Thomas 
Graham,  was  so  impressed  by  the  differences  between  typical 
colloids  and  typical  molecular  dispersoids  that  he  declared  the 
two  to  represent  "different  worlds  of  matter."  He  endeavored 
in  consequence  to  contrast  them  as  much  as  possible.  The  ma- 
jority of  his  successors  followed  him  in  this,  and  only  recently 
has  the  effort  been  made  to  cease  discovering  rare  and  sharp 
distinctions  between  colloids  and  molecular  dispersoids.  As  a 
matter  of  fact  no  such  sharp  distinctions  exist.  But  the  realiza- 
tion of  this  fact  was  important  in  that  it  yielded  a  new  point  of 
view,  on  the  basis  of  which  it  became  possible  to  formulate  the 
concept  of  the  dispersoid,  and  with  it  to  obtain  a  rational 
systematization  of  these  bodies1  (see  later).  It  must  be  em- 
phasized, however,  that  even  today  comparatively  few  investi- 
gations are  carried  out  with  the  conscious  purpose  of  studying 
these  transition  phenomena,  more  especially  the  changes  which 
the  individual  physical  and  physico-chemical  properties  exhibit 
with  progressive  variations  in  the  degree  of  dispersion. 

In  some  forthcoming  chapters  of  this  book  (Part  V,  on  the 
History  of  Colloid- chemistry)  we  shall  call  attention  to  many 
of  these  transition  phenomena. 

1  See  Wo.  Ostwald,  Koll.-Zeitschr.  i,  291,  331  (1907). 


4<D  GENERAL  COLLOID-CHEMISTRY 

§7.  General  Colloid-chemical  Nomenclature 

Dispersoids  are  called  sols  if  they  have  the  properties  de- 
scribed in  the  foregoing  paragraphs,  if  their  degree  of  dispersion 
lies  between  6.io5  and  6.io7,  and  the  disperse  phase  is  uniformly 
distributed  throughout  the  dispersion  means.  This  name  origi- 
nated with  Thomas  Graham.1  When  we  speak  of  "a  colloid" 
we  nearly  always  mean  one  in  this  condition,  in  other  words, 
one  in  the  sol  condition.  There  exists  also  what  Graham  first 
called  the  gel  condition.  A  sol  becomes  a  gel  when  its  degree  of 
dispersion  is  decreased  in  such  a  way  that  it  passes  beyond  the 
lower  limits  characteristic  of  the  colloids,  in  other  words,  when 
the  system  becomes  microscopically  heterogeneous.  A  usual, 
though  not  an  absolutely  necessary  accompanying  phenomenon  of 
gel  formation  is  a  loss  of  the  uniform  distribution  of  the  disperse 
phase  in  the  dispersion  means.  The  sol  "precipitates"  "clots" 
"coagulates"  "cements"  etc.  It  is  sometimes  said  that  the  sol 
"gelatinizes"  but  it  is  best  to  reserve  this  word  for  another  process 
which  can  be  distinguished  from  the  ordinary  "precipitation" 
"clotting"  or  "coagulation." 

The  phenomena  opposed  to  "coagulation,"  in  other  words, 
those  which  result  in  an  increase  of  the  degree  of  dispersion  and 
tend  toward  an  approximately  or  absolutely  uniform  distribution  of 
the  disperse  phase  in  the  dispersion  means  are  summed  up  under 
the  term  "peptization."  This  term  was  also  first  suggested  by 
Graham.  All  variations  in  the  degree  of  dispersion  and  in  the 
properties  connected  with  it  are  designated  by  a  term  introduced 
by  Wolfgang  Pauli — "changes  of  state  in  colloid  systems."  When 
a  change  in  the  state  of  a  colloid  may  be  reversed  by  reversing 
the  conditions  which  brought  that  change  about,  it  is  said  to  be 
"reversible."  Thus  when  a  colloid  which  has  been  precipitated 
by  a  salt  goes  back  into  solution  on  removal  of  the  salt,  the 
colloid  change  is  said  to  be  "reversible."  On  the  other  hand,  if 
this  does  not  occur  it  is  "irreversible."  The  reversibility  of  such 
a  change  of  condition  is  not  determined,  in  the  main,  by  the 
nature  of  the  colloid  itself,  but  rather  by  the  character  of  the 
conditions  which  produce  the  coagulation.  Thus  the  precipitation 
of  typical  protein  sols  by  neutral  salts  is  reversible,  but  their  pre- 

1Th.  Graham,  Phil.  Trans.  Roy.  Soc.  (1861);  Liebig's  Ann.,  121,  i  (1862). 


GENERAL   CONSTITUTION   OF    COLLOID    SYSTEMS  41 

cipitation  by  heat  is  irreversible.  We  cannot  therefore  speak  of 
reversible  and  irreversible  colloids,  as  is  still  frequently  done,  but 
only  of  reversible  and  irreversible  changes  in  state  in  the  colloids. 
Another  inappropriate  word  is  "solid-sol,"  by  which  is  really  meant 
a  gel  which  will  redissolve  in  the  dispersion  means  from  which  it 
has  been  precipitated  or  dried. 

According  to  the  chemical  name  of  the  dispersion  means  we 
also  distinguish  between  hydrosols  and  hydrogels,  alcosols  (alcohol- 
sols)  and  alcogels,  sulphosols  (sulphuric  acid  sols)  and  sulphogels. 
If  the  dispersion  means  is  an  organic  liquid  the  dispersoid  is  called 
an  organosol,  etc.  The  chemical  name  of  the  disperse  phase  is 
used  as  a  prefix,  thus:  gold-hydrosol,  silicic  acid-alcogel,  ice- 
zylosol,  etc. 


CHAPTER  II 

RELATIONS  BETWEEN  THE  PHYSICAL  STATE  AND  THE 
GENERAL  PROPERTIES  OF  COLLOID  SYSTEMS 

§8.  Classification  of  Dispersoids  According  to  the  State  of  their 

Phases 

i.  The  Physical  State  of  the  Disperse  Phase  as  a  Principle  of 
Classification. — As  soon  as  we,  attempt  to  carry  out  practically 
the  classification  of  the  dispersoids  on  the  basis  of  their  degree  of 
dispersion  we  find  that  this  principle  does  not  always  suffice.  Two 
dispersoids,  for  example,  may  be  identical  in  that  the  size  of  the 
particles  of  their  disperse  phases  is  the  same,  yet  their  other 
properties  may  differ  so  widely  that  the  similarity  appears  as  a 
mere  incident.  This  may  be  illustrated  by  the  great  difference 
between  a  quartz  or  kaolin  suspension  in  water  and  an  emulsion 
of  oil  in  water  even  when  the  average  size  of  the  particles  in  the 
two  is  the  same.  But  it  is  for  the  colloids  in  particular  that  such 
a  classification  according  to  degree  of  dispersion  seems  to  be 
entirely  inadequate.  First,  the  extremes  between  which  the 
degree  of  dispersion  may  vary  in  colloids  are  not  widely  separated. 
According  to  the  classification  of  Zsigmondy  the  degree  of  dis- 
persion in  colloids  can  only  vary  between  6.io5  and  6.io7  while 
that  of  coarse  dispersions  and  molecular  and  supermolecular 
dispersoids  is  limited  on  one  side  only.  Second,  different  colloid 
solutions  are  known  in  which  the  particles  are  of  the  same  size, 
but  which  in  other  points  differ  so  markedly  from  each  other 
that  the  similarity  in  degree  of  dispersion  seems  merely  accidental 
or  unimportant.  What  we  need  therefore  is  an  additional 
principle  of  classification  which  is  not  based  upon  the  size  of  the 
particles.  Such  an  one  lies  close  at  hand. 

Since  colloid  systems  are  heterogeneous  or  polyphasic — a 
fact  discussed  in  detail  in  the  preceding  chapter — we  may 
use  the  physical  character  (the  physical  state)  of  the  phases 
composing  the  systems  as  a  basis  for  classification.  We  may 

42 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  43 

classify  dispersoids  according  to  the  character  of  their  phases  quite 
as  justly  as  according  to  the  number  or  the  degree  of  dispersion 
of  their  phases.  Theoretically,  such  a  classification  would  be  as 
valuable  as  the  other  two.  But  when  we  consider  that  an  entire 
series  of  properties  change,  whenever  the  physical  character  of  a 
phase  changes,  a  classification  based  on  this  character  is  evidently 
a  more  natural  one  than  that  based  upon  any  arbitrarily  chosen 
single  property.  The  succeeding  paragraphs  will  show  that  a 
classification  of  dispersoid  systems,  more  particularly  of  colloid 
systems,  according  to  the  physical  state  of  their  disperse  phases  is 
at  least  as  important  as  their  classification  on  the  basis  of  their 
degree  of  dispersion. 

2.  Classification  of  the  Dispersoids  According  to  the  Physical 
State  of  Their  Phases.- — The  most  important  dispersoids  that 
need  to  be  considered  are  diphasic.  By  joining  the  three  physical 
states  of  matter  in  pairs  we  obtain  the  following  possibilities: 

1.  Solid  +  Solid.      4.  Liquid  +  Solid.          7.  Gas  +  Solid. 

2.  Solid  +  Liquid.   5.  Liquid  +  Liquid.        8.  Gas  +  Liquid. 

3.  Solid  +  Gas.        6.  Liquid  +  Gas.  9.  (Gas  +  Gas.) 

No  example  exists  of  a  dispersoid  having  the  composition 
Gas  +  Gas,  for  gases  are  freely  and  completely  miscible  with  each 
other  in  all  proportions.  Examples  of  the  other  classes  are: 

1.  Solid  +  Solid. — Intercalations  of  foreign  particles  in  many 
minerals  (microliths,  etc.),  carbon  particles  in  iron,  of  coloring 
matter  in  mineral  salts  and  precious  stones;  "solid"  colloid  solu- 
tions, mixed  crystals,  solid  solutions. 

2.  Solid  +  Liquid. — Liquid  intercalations  in  many  minerals, 
water  of  occlusion,  inclusion  and  crystallization. 

3.  Solid  +  Gas. — Gaseous  inclusions  in  many  minerals  (meer- 
schaum, pumice  stone,  lava,  tufa),  solutions  of  gases  in  solids 
(hydrogen  in  iron,  etc.) . 

7.  Gas  +  Solid. — Smoke,   for  example  tobacco  smoke;  con- 
densing metallic  vapors  (F.  Ehrenhaft);    cooling  vapors  of  am- 
monium chloride;  cosmic  dust,  etc. 

8.  Gas  +  Liquid. — The  fog  formed  at  the  liquefaction  point  of 
gases  or  in  the  condensation  of  steam;  atmospheric  fog,  clouds, 
Tyndall's  photochemically  produced  liquid  fog,  etc. 

All  these  will  be  discussed  in  detail  later. 


44  GENERAL  COLLOID-CHEMISTRY 

The  classes  of  dispersoids  mentioned  under  4,  5,  and  6  are  by 
far  the  most  important  and  deserve  special  attention  in  colloid 
chemistry.  The  dispersion  means  is  liquid  in  all  three  cases,  the 
disperse  phase  is  solid  in  the  first  instance,  liquid  in  the  second,  and 
gaseous  in  the  third.  The  most  familiar  examples  of  these  three 
classes  of  dispersoids  are  the  coarse  dispersions  known  as  suspen- 
sions, emulsions,  and  foams. 

§9.  Transition  Phenomena.    Complex  Dispersoids 

i.  General  Considerations.  Influence  of  Temperature  and 
Degree  of  Dispersion. — As  soon  as  we  study  the  problem  closely, 
it  becomes  evident  that  transitions  are  encountered  in  this  scheme 
of  classification  also.  This  is  more  particularly  true  in  passing 
from  the  solid  to  the  liquid  state,  a  transition  which  is  produced 
more  frequently  and  more  easily,  as  is  well  known,  than  the  transi- 
tion from  liquid  to  gas.  Thus  one  and  the  same  dispersoid  may 
be  an  emulsion  or  a  suspension,  depending  upon  the  temperature. 
At  ordinary  temperatures  a  few  drops  of  an  alcoholic  mastic 
solution  or  of  an  alcoholic  solution  of  rosin  form  a  suspension  when 
poured  into  an  excess  of  water,  but  if  the  system  be  heated  to  the 
melting  point  of  the  resins  a  mixture  of  two  liquid  phases  or  an 
emulsion  results.  Between  these  two  temperatures  all  possible 
transitions  between  solid  and  liquid  may  appear.  In  the  last 
analysis  the  difficulty  of  drawing  a  sharp  line  between  suspensions 
and  emulsions  is  identical  with  the  more  general  one  of  formulating 
a  precise  definition  of  the  solid  as  contrasted  with  the  liquid  state 
of  matter.  Since  the  discovery  of  liquid  crystals  and  crystalline 
liquids  the  only  criteria  we  seem  to  have  left  to  characterize  the 
solid  state  are  its  enormous  internal  friction  and,  particularly,  the 
absence  of  free  (positive)  surface  tension  which  under  ordinary 
conditions  determines  the  shape  of  a  liquid. 

When  we  deal  with  droplets  of  microscopic  size,  their  liquid 
character  [that  is  to  say,  the  presence  of  free  (positive)  surface 
energy  in  them]  can  be  demonstrated  rather  easily  by  deformation 
experiments.  But  it  is  much  harder  to  ascertain  the  physical 
state  of  a  disperse  phase  when  higher  dispersion  values  come  into 
play.  As  E.  Hatschek1  has  shown  particularly  well,  the  amount 

*E.  Hatschek,  Koll.-Zeitschr.,  7,  81  (1910). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  45 

of  energy  necessary  to  produce  the  deformation  of  a  liquid  droplet 
increases  very  rapidly  with  increase  in  degree  of  dispersion. 
While  its  own  weight  suffices  to  squeeze  a  macroscopic  oil  globule 
through  a  glass  tube  half  its  own  diameter  a  pressure  of  4.5  atmos- 
pheres is  necessary  under  analogous  experimental  conditions 
if  the  oil  droplet  is  0.2^  in  diameter.  Such  a  drop  is  still  visible 
under  the  microscope.  The  explanation  for  this  lies  in  the  great 
increase  in  the  surface  energy  of  a  given  volume  with  its  progressive 
subdivision.  (For  details  see  later.)  The  greater  the  degree  of 
subdivision  of  a  liquid  disperse  phase,  the  more  does  it  approximate 
a  solid  in  its  mechanical  behavior. 

P.  P.  von  Weimarn,1  starting  with  other,  mainly  moleculo- 
kinetic,  conceptions,  has  reached  a  conclusion  the  converse  of  this. 
The  properties  of  solid  disperse  particles  approach  those  of  a 
liquid  as  their  degree  of  dispersion  increases.  We  may  say, 
therefore,  that  property  differences  between  liquid  and  solid  disperse 
phases  become  progressively  less  marked  as  the  degree  of  dispersion 
increases.  In  entire  agreement  with  this  conclusion  is  the  fact 
that  the  importance  of  the  original  physical  state  disappears 
entirely  when  the  phases  become  molecular-disperse.  Thus  all 
solutions  of  acetic  acid  are  the  same  whether  they  are  prepared 
from  the  vapor  or  from  the  liquid  or  solid  forms  of  acetic 
acid.2 

2.  Influence  of  Concentration  upon  State  in  Complex  Dis- 
persoids. — Further  analysis  shows  that  in  addition  to  tempera- 
ture and  degree  of  dispersion  the  concentration)  that  is  to  say  the 
quantitative  relation  of  the  components  of  a  dispersoid  to  each 
other,  may  have  an  important  influence  upon  its  state.  Two 
possibilities  may  be  distinguished: 

1.  The  state  of  the  dispersoid  as  a  whole  may  vary  with  the 
concentration. 

2.  The  state  of  the  individual  phases  of  the  system  may  vary 
with  the  concentration. 

1  P.  P.  von  Weimarn,  Koll.-Zeitschr.,  6,  32  (1910);  7,  155  (1910). 

2  It  is  therefore  impossible  and  so  purposeless  to  try  to  distinguish  molecularly  dis- 
perse systems  (solutoids)  from  each  other  on  the  basis  of  the  "solid"  or  "fluid" 
nature  of  the  disperse  phase  as  P.  P.  von  Weimarn  (I.e.)  proposes.     But  this  state- 
ment is  not  to  be  construed  as  meaning  that  the  processes  of  formation  or  of  solution  are 
identical  in  the  two  cases.    The  heats  of  solution  are  certainly  different.     Since  in 
a  classification  of  disperse  systems  we  only  classify  the  like  results  of  very  different 
physical  and  chemical  processes,  the  advantage  of  utilizing  these  differences  in  modes 
of  preparation  for  purposes  of  classification  is  not  evident. 


46  GENERAL  COLLOID-CHEMISTRY 

We  seem  at  first  sight  to  deal  here  with  rare  and  complex 
phenomena,  but  while  it  is  true  that  only  suggestions  of  them 
are  found  among  the  best-known  dispersoids,  namely  the  mo- 
lecularly  and  coarsely  disperse,  they  play  an  important  part  in 
certain  colloids.  Let  us  first  consider  the  behavior  of  the  simpler 
dispersoids  of  this  type  when  their  concentration  is  varied. 

(a)  The  state  of  a  molecular-disperse  solution  is  determined 
by  the  state  of  the  solvent.  The  reason  for  this  is  that  the  original 
state  of  the  dissolved  substance  has  lost  its  importance  when  a 
molecular-disperse  phase  results,  as  already  explained  above.  In 
the  case  of  those  atypical  solutions  which  are  more  or  less  solid, 
there  are  good  reasons  for  supposing  that  in  them  the  disperse 
phase  has  not  remained  in  a  molecular  state  of  subdivision,  but 
has  been  polymerized,  associated  or  condensed  into  submolecular 
and  then  into  colloid  particles,  as  discussed  above.  Thus  soap 
solutions  are  molecular-disperse  and  liquid  in  low  concentrations, 
but  are  colloid  and  solid  in  higher  concentrations.  If  the  con- 
centration is  varied  while  the  molecular  degree  of  dispersion  is 
maintained  a  separation  of  the  disperse  phase  in  liquid  or  solid 
form  generally  occurs,  in  other  words  saturation  is  attained. 

(6)  The  state  of  a  coarsely  disperse  system  depends  in  an 
interesting  way  upon  concentration.  In  certain  extremes  of 
concentration,  bodies  having  a  liquid  dispersion  means  and  a  gase- 
ous, liquid,  or  solid  disperse  phase  may  assume  some  of  the 
properties  of  solids,  such  as  constancy  of  form  and  elasticity. 
Solid  powders  mixed  with  a  little  water  illustrate  this.  Sand 
with  a  definite  but  by  no  means  immeasurably  small  water 
content  may  be  cut  into  slices.  Emulsions  of  mineral  oil  in  soap, 
or  of  water  in  mineral  soaps  form  sectionable  pastes  in  certain 
concentrations;1  if  the  concentrations  are  slightly  changed  a 
coarse  mixture  of  liquids  is  again  formed  which  may  be  poured 
from  one  vessel  to  another.  As  is  well  known,  foams  prepared 
with  small  quantities  of  a  liquid  dispersion  means  may  have  a 
definite  shape  and  exhibit  considerable  solidity.  Thus,  a  slice  of 
pasteboard  weighing  two  grams  will  not  sink  into  a  well-beaten 
saponin  foam. 

It  is  characteristic  of  all  these  systems  that   they  contain 

a  great  excess  of  the  disperse  phase,  though  even  here  an  optimum 

1  See  S.  U.  Pickering,  Koll.-Zeitschr.,  7,  n  (1910);  D.  Holde,  ibid.,  3,  270  (1906). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  47 

concentration  exists  beyond  which  the  system  again  loses  its  solid 
state.1  In  these  "highly  concentrated  dispersoids"  the  dispersion 
means  surrounds  the  disperse  phase  with  a  "liquid  film"  which 
deprives  the  disperse  particles  of  their  mobility,  and  so  imposes 
upon  them  the  properties  of  a  solid. 

(c)  Particularly  interesting  and  important  phenomena  appear 
among  the  complex  dispersoids.  The  more  important  character- 
istics of  these  bodies,  such  as  variations  in  concentration  in  the 
individual  dispersoids  with  temperature  and  total  concentration, 
and  variations  in  the  degree  of  dispersion  with  the  same  factors, 
were  mentioned  above.  In  considering  the  influence  of  con- 
centration on  the  state  of  complex  dispersoids  we  have  therefore 
to  deal  with  an  exceptionally  large  number  of  factors  which  may 
all  act  in  the  same  general  direction  but  which  may  also  serve  to 
antagonize  each  other. 

A.  COMPLEX  SYSTEMS  HAVING  THE  COMPOSITION 
LIQUID  +  LIQUID 

(a)  Influence  of  Concentration  upon  the  State  of  the  Dis- 
persoid  as  a  Whole.— In  Liquid  +  Liquid  systems  the  same  in- 
fluence of  concentration  described  above  for  simple,  coarsely 
disperse  suspensions  may  exhibit  itself,  and  a  body  having  the 
properties  of  a  solid  may  result.  Here  also  a  sudden  " setting" 
may  take  place  in  certain  limited  regions  of  concentration.  But 
to  this  there  must  be  added  the  influence  of  the  total  concentra- 
tion not  alone  upon  the  concentration  of  the  individual  dispersoids, 
and  consequently  upon  their  physical  properties,  but  also  upon  the 
degree  of  dispersion  of  the  system  as  a  whole,  and  thus  upon  its 
state.  If  we  try  to  imagine  what  must  be  the  effect  of  these  in- 
dividual factors  upon  the  behavior  of  such  complex  dispersoids 
we  can  only  suspect  that  the  changes  in  state  of  the  system  as  a 
whole  with  changes  in  concentration  or  temperature  must  be 
smoother  in  the  case  of  the  complex  dispersoids  than  in  the  case 
of  the  simple  ones.  For,  generally  speaking,  it  is  probable  that 
the  consequences  of  any  change  will  manifest  themselves  less 
clearly  when  a  great  number  of  factors  working  partly  in  the 
same,  partly  in  opposite  directions,  are  affected  by  it  than  when 
only  one  or  a  relatively  small  number  of  such  determining  factors 

1  See  Wo.  Ostwald,  Koll.-Zeitschr.,  6,  185  (1910). 


48  GENERAL  COLLOID-CHEMISTRY 

are  affected.  This  is  particularly  true  of  systems  that  occupy  a 
middle  position  among  the  dispersoids,  in  other  words  of  the 
colloids.  It  follows  from  the  principle  of  continuity  which  justly 
plays  a  great  role  in  all  science  that  colloid  systems  having  the 
composition  Liquid  +  Liquid  must  occupy  a  middle  position  be- 
tween coarse  dispersions  and  molecular  dispersoids  in  this  respect 
also. 

(b)  Influence  of  Concentration  on  the  State  of  the  Disperse 
Phase.^  —  Exceptionally  complicated  and  interesting  relationships 
become  apparent  when  we  take  into  consideration  the  fact  that  in 
complex  systems  the  state  of  aggregation  of  one  of  the  individual 
dispersoids,  the  disperse  phase  for  example,  may  be  changed  in 
the  same  way  as  that  of  the  compound  dispersoid  by  a  change]  in 
concentration.  For  there  is  no  reason  for  excluding  the  possi- 
bility that  a  floating  droplet  having  the  composition  Liquid  + 
Liquid  may  "set"  in  certain  concentrations  and  at  certain  tem- 
peratures as  does  the  dispersoid  as  a  whole  at  other  concentrations, 
perhaps.  Just  as  soap,  water,  and  paraffin  oil  may  form  a  body 
having  the  properties  of  a  solid  under  certain  conditions,  so  a 
disperse  particle  having  a  similar  composition  may  suddenly 
stiffen  even  though  the  dispersion  means  itself  may  still  be  liquid. 
A  great  variety  of  possibilities  exist  here  which  we  shall  encounter 
again  later. 


B.  COMPLEX  DISPERSOIDS  HAVING  THE  COMPOSITION      ] 
LIQUID  +  SOLID 

(a)  Influence  of  Concentration  on  the  State  of  the  Dispersoid 
as  a  Whole.  —  It  is  easily  seen  that  when  a  complex  system  has 
the  composition  Liquid  +  Solid  (a  suspension  of  swollen  gelatine 
particles,  for  example)  it  may  assume  the  properties  of  a  solid  as 
its  concentration  rises.  There  need  only  be  enough  particles  in 
a  given  volume  of  liquid  so  that  in  the  process  of  swelling  they 
interfere  with  each  other's  movement  in  order  to  get  a  stiff  jelly. 
Figuratively  speaking,  the  particles  at  higher  concentrations 
struggle  with  each  other  for  the  dispersion  means,  and  as  the 
amount  of  liquid  at  their  disposal  decreases  the  particles  adhere 
more  and  more  firmly  to  each  other.  This  may  be  observed  ex- 
perimentally if  the  liquid  dispersion  means  is  gradually  removed 


GENERAL  PROPERTIES   OP   COLLOID    SYSTEMS  49 

by  evaporation.  In  this  way  a  typical  solid  is  finally  obtained.1 
Conversely,  such  a  complex  system  will  tend  to  approach  a  normal 
liquid  in  character  as  the  number  of  suspended  particles  con- 
tained in  the  unit  volume  is  lessened.  If  the  dispersion  means 
is  also  heterogeneous,  that  portion  of  it  which  is  made  up  of  the 
disperse  phase  must  also  increase  when  the  total  concentration 
is  increased  so  that  the  augmented  viscosity  resulting  from 
this  will  further  aid  in  giving  the  dispersoid  as  a  whole  a  solid 
consistency. 

(b)  Influence  of  Concentration  on  the  State  of  the  Disperse 
Phase.- — The  state  of  the  solid  disperse  phase  more  nearly  ap- 
proaches that  of  a  liquid  the  greater  the  amount  of  dispersion 
means  it  has  absorbed. 

To  sum  up  we  may  say  that  a  great  variety  of  relations  exists 
between  the  state  of  the  individual  phases  composing  the  dis- 
persoid and  its  general  properties.  Of  greatest  significance  is 
the  fact  that  a  system  having  the  properties  of  a  solid  may  be  formed 
from  a  disperse  mixture  of  non-solid  phases.  Other  disperse 
systems  having  the  composition  Gas  +  Solid  or  Gas  +  Liquid 
may  assume  the  properties  of  liquids.  Thus  smoke  or  fog  some- 
times exhibit  phenomena  of  (positive)  surface  tension  (" rings" 
of  smoke) ;  and  at  least  most  of  the  hydrostatic  properties  of 
liquids  may  be  easily  demonstrated  in  extremely  concentrated 
systems  of  this  composition,  as  in  not  too  fine,  dry  sand. 

§10.  Colloid  Systems  as  Suspensoids  and  Emulsoids 

i.  General  Considerations. — Once  we  accept  the  physical 
heterogeneity  of  colloid  systems  we  are  compelled  to  consider  the 
state  of  their  phases.  The  dispersoids  that  must  first  be  dealt 
with  are  those  which  have  a  liquid  dispersion  means.  Special 
attention  must  therefore  be  paid  to  the  part  played  by  the  state  of 
the  disperse  phase. 

In  considering  the  three  types  of  systems  which  belong  to  this 
group  we  may  disregard  the  class  Liquid  +  Gas,  for  these  (foams 
composed  of  extremely  small  bubbles)  are  not  typical  representa- 
tives of  the  colloids.  This  should  not  be  taken  to  mean  that  such 
systems  are  unknown  or  are  incapable  of  existing.  The  turbidity 
which  appears  in  liquids  about  the  critical  vaporization  point  is 

1  The  crystalline  structure  of  solid  bodies  is  disregarded  here. 
4 


50  GENERAL  COLLOID-CHEMISTRY 

probably  dependent  upon  the  formation  of  a  great  number  of  gas 
bubbles  in  a  high  degree  of  dispersion.  A  detailed  compari- 
son of  the  properties  of  these  systems  with  those  of  others 
has  not  been  made  as  yet,  although  their  optical  behavior,  the 
effect  of  the  walls  of  the  vessels  containing  them,  etc.,  demonstrate 
the  close  relationship  of  these  to  the  dispersoids.  A  detailed 
theoretical  and  practical  study  of  the  whole  problem,  more 
especially  of  the  critical  cloudiness  of  liquid  mixtures,  would  yield 
fruitful  results,  for  great  similarities  have  already  been  shown 
to  exist  between  these  systems  and  certain  colloids  (see  later). 

We  need  to  concern  ourselves  therefore  only  with  colloid 
solutions  having  the  composition  Liquid  -f  Solid  and  Liquid  + 
Liquid.  According  to  our  classification  we  may  expect  to  en- 
counter two  classes  of  colloids,  and  our  problem  narrows  itself 
down  to  the  relations  existing  between  suspensions  and  emulsions 
on  the  one  hand  and  colloid  solutions  on  the  other.  On  a  priori 
grounds  it  would  seem  possible  that  the  coarse  dispersions  men- 
tioned might  yield  two  types  of  colloid  solutions  when  the  degree 
of  dispersion  is  properly  increased.  The  question  as  to  what 
properties  such  systems  exhibit  is  worth  our  attention. 

2.  The  Empirical  Establishment  of  Two  Classes  of  Colloids.— 
The  existence  of  two  classes  of  colloids  differing  markedly  from 
each  other  has  recently  become  apparent  on  purely  empirical 
grounds  and  entirely  independently  of  any  theoretical  considera- 
tions. Protein  and  gelatine  solutions  represent  one  extreme, 
Zsigmondy's  aqueous  gold  dispersoids  the  other  of  these  two  types 
of  colloid  solutions.  Different  names  are  employed  in  the  literature 
for  these  two  sets  of  colloids,  different  investigators  having  made 
use  of  different  of  their  special  properties  for  their  characteriza- 
tion. V.  Henri1  calls  them  "stabile"  and  "instabile"  colloids, 
A.  A.  Noyes,2  "colloidal  solutions"  and  "colloidal  suspensions," 
J.  Perrin,3  "  hydrophilic "  and  "  hydrophobic "  colloids,  and  also 
"hydrosoles  stables  et  colloides  hydrophiles."  After  Wolfgang  Ost- 
wald4  pointed  out  that  the  latter  names  are  too  narrow  since  the 
character  of  the  dispersion  means  may  vary,  H.  Freundlich5  and  W. 

1  V.  Henri,  Zeitschr.  f.  physik.  Ch.,  51,  29  (1905). 

2  A.  A.  Noyes,  Journ.  Amer.  Chem.  Soc.,  27,  85  (1905). 

3  J.  Perrin,  J.  Chim.  Phys.,  3,  50  (1905). 

4  Wo.  Ostwald,  Koll.-Zeitschr.,  I,  291,  331  (1907). 

5  H.  Freundlich  and  W.  Neumann,  Koll.-Zeitschr.,  3,  80  (1908). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  51 

Neumann  suggested  the  more  comprehensive  terms  "lyophilic" 
and  "lyophobic"  colloids.  All  these  terms  are  either  based  upon 
the  existence  of  some  striking  individual  difference  between  the  two 
groups  or  else  are  expressive  of  particular  theoretical  conceptions. 
Noyes  has  analyzed  the  two  classes  of  colloids  from  a  broader 
experimental  point  of  view.  He  characterizes  one  type  as 
"viscous,  gelatinizing,  colloidal  mixtures  not  (easily)  coagulated  by 
salts"  the  other  as  "non-viscous,  non-gelatinizing,  but  easily 
coagulable  mixtures"  A  possible  addition  to  Noyes'  happy  em- 
pirical description  is  that  the  former  usually  have  a  lower  surface 
tension  than  their  pure  dispersion  means,  while  in  the  latter  the 
tension  is  practically  unchanged.  Further,  electrical  factors 
usually  play  a  more  important  r61e  in  the  latter  than  in  the 
former.  This  is  evidenced,  for  example,  by  the  great  precipitat- 
ing power  which  polyvalent  ions  have  upon  the  non-viscous 
colloids. 

Examples  of  the  "non- viscous,  etc.,"  colloids  are  the  metallic 
,sols,  sulphide  sols,  many  dyes  (congo-red  for  instance),  iron 
hydroxide  in  dilute  solution,  etc.  The  best  illustrations  of  the 
other  class  are:  the  proteins  and  related  substances,  gelatine,  agar, 
cholesterol,  silicic  acid,  "  me ta  "-phosphoric  acid,  stannic  acid, 
meta-hydroxides  in  concentrated  solution,  so-called  gelatinous 
salts  (sulphates,  phosphates,  carbonates,  etc.),  dye-stuffs  like 
night-blue,  etc. 

The  question  now  arises  whether  these  two  classes  of  colloid 
solutions  do  actually  represent  dispersoids  having  the  composition 
Liquid  +  Solid  and  Liquid  +  Liquid. 

3.  The  Theoretical  Characterization  of  the  Two  Classes  of 
Colloids. — Even  the  theoretical  conceptions  which  led  to  the  use 
of  the  expressions  alyophilic"  and  " lyophobic"  imply  the  exist- 
ence of  a  close  relationship  between  the  properties  denoted  by  these 
names  and  the  state  of  the  disperse  phase.  This  is  particularly 
true  of  the  "lyophilic"  colloids  which  we  characterize  as  systems 
having  the  composition  Liquid  +  Liquid,  for  when  we  say  that 
the  disperse  phase  is  here  composed  of  "  exceedingly  swollen  par- 
ticles" or  of  "particles  united  to  a  great  number  of  liquid  mole- 
cules," we  imply  that  its  state  is  liquid.  Statements  like  this  of 
J.  Perrin:1  "Un  granule  d'hydrosol  stable  contiendrait  une 

1  J.  Perrin,  I.e.,  84,  87. 


52  GENERAL  COLLOID-CHEMISTRY 

tres  forte  proportion  d'eau,  90  per  cent,  par  exemple"  can  hardly 
be  interpreted  differently.  Yet  while  such  statements  may  be 
regarded  as  approaching  a  characterization  of  the  two  classes  of 
dispersoids  on  the  basis  of  the  state  of  their  disperse  phases,  a 
detailed  analysis  of  these  two  classes  based  on  experimental  study 
was  attempted  only  recently.  This  is  particularly  true  of  the 
"ly'ophilic"  colloids.  The  colloids  described  as  "lyophobic," 
''unstable,"  etc.,  were  characterized  as  dispersoids  having  the 
composition  Liquid  +  Solid  early  in  the  history  of  colloid-chemical 
investigation  as  an  almost  self-evident  conclusion  to  be  drawn  from 
a  consideration  of  their  properties.  The  similarities  between 
colloid  metals,  for  example,  and  coarse  dispersions  having  the 
composition  Liquid  +  Solid  are  so  great  and  the  relations  between 
them  so  intimate  and  striking  that  even  before  the  formulation 
of  the  concept  of  "  colloidality,"1  B.  J.  Richter,2  M.  Faraday,3 
and  J.  Berzelius4  thought  of  the  former  as  suspensions  of  particles 
of  the  same  character  as  "reguline"  metals.  B.  J.  Richter  showed 
as  far  back  as  1802  that  gold  in  its  well-known  "  aqueous  solu- 
tions "  does  not  exist  here  in  some  unknown  molecular  condition, 
but  in  a  finely  divided  metallic  (solid)  state.  This  conclusion 
was  afterwards  confirmed  by  Faraday  and  Zsigmondy,  and  ex- 
tended to  other  colloids  of  this  group  by  other  investigators. 

We  might  now  try  to  correlate  these  two  empirically  estab- 
lished types  of  colloid  systems  with  the  dispersoids  having  the  com- 
position Liquid  +  Solid  and  Liquid  +  Liquid  by  detailing  the 
similarities  between  the  two  sets  of  systems.  But  such  a  com- 
parison would  presuppose  knowledge  of  a  great  number  of  the 
special  properties  of  colloid  systems  which  only  the  succeeding 
portions  of  this  book  will  bring.  The  results  of  comparison  would 
not  be  convincing,  for  the  facts  to  be  employed  could  only  be 
pointed  out  here  in  brief.  We  shall  therefore  for  the  present  only 
assume  that  the  two  types  of  colloid  systems  under  discussion 

1  Wo.  Ostwald,  Koll.-Zeitschr.,  I,  291,  331   (1907).     The  statement  of  A.  Midler, 
[Allg.  Chemie  d.  Koll.,  147,  186,  Leipzig  (1907)],  that  G.  Quincke  [Ann.  d.  Physik. 
(4)  9>  797)  1009,  etc.  (1907)]  first  classified  colloids  according  to  the  type  of  the  dis- 
perse phase  is  not  correct.     The  statements  of  Quincke  which  no  doubt  led  to  this 
historical  error,  do  not  refer  to  colloid   systems   but   to   coarsely  dispersed  ones. 
Quincke  holds  all  colloid  solutions  (including  those  of  arsenious  trisulphides)  to  be 
mixtures   of   two  fluid  phases  (I.e.,  1009,    1034,  etc.).     See  in  this  connection  G. 
Bredig  [Ann.  d.  Physik.,  4,  n,  221  (1903)]. 

2  B.  J.  Richter,  see  Wilh.  Ostwald,  Koll.-Zeitschr.,  4,  5  (1909). 

3  M.  Faraday,  Philos.  Mag.  (4)  14,  401,  512  (1859). 

4  J.  Berzelius,  Lehrb.  d.  Chemie,  2  Aufl.,  2,  244  (1823). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  53 

differ  from  each  other  in  the  matter  of  the  state  of  their  disperse 
phases  and  in  the  properties  which  result  from  this  fundamental 
difference.  We  shall  of  course  not  disregard  the  necessity  of 
proving  this  assumption  later.1 

4.  The  Frequency  of  Occurrence  of  Complex  Emulsoids.— 
Let  us  anticipate  a  particularly  important  generalization  which 
follows  from  characterizing  the  "lyophilic"  colloids  as  systems 
having  the  composition  Liquid  +  Liquid.  The  behavior  of 
such  colloids  demonstrates  clearly  the  liquid  character  of  the 
disperse  phase  in  them  and  emphasizes,  first,  that  the  "lyophilic" 
colloids  are  complex  dispersoids,  that  is  to  say,  their  individual 
phases  are  in  themselves  dispersoids  of  a  higher  degree  of  dis- 
persion, and,  second,  that  the  composition  of  these  individual 
dispersoids  as  well  as  their  degree  of  dispersion  varies  greatly 
with  concentration,  temperature,  etc.  Further,  in  consequence 
of  the  complex  character  of  these  systems  the  state  of  the  disperse 
phase  may  pass  progressively  from  liquid  through  semisolid  to  solid 
and  back  again.  The  possibilities  of  variation,  in  the  state  of  a 
complex  dispersoid  with  degree  of  dispersion,  temperature, 
concentration,  etc.,  as  discussed  on  p.  45,  appear  very  clearly 
in  these  colloids  and  undoubtedly  constitute  one  of  the  chief 
reasons  why  their  behavior  is  so  much  more  varied  than  that  of 
colloids  having  the  composition  Liquid  +  Solid.2  The  value  of 
this  conception  will  also  show  itself  in  discussing  the  individual 
phenomena  characteristic  of  these  systems,  when  it  will  be- 
come evident  that  the  different  theoretical  views  held  regarding  the 
properties  of  these  two  classes  of  colloids  may  not  only  be  corre- 

1  An  exact  classification  and  description  of  both  classes  of  colloids  according  to  the 
type  of  the  phases  was  indicated  in  the  first  edition  of  this  book.     Since  then  I  have 
found  only  further  and  very  excellent  support  for  this  view.     I  hope  in  the  near  future 
to  publish  a  monograph  on  the  physical  theory  of  colloids  of  the  composition  Liquid + 
Liquid  (see  the  following  footnote). 

2  Even  in  the  first  edition  of  this  book  (pp.  in,  328,  356,  374,  etc.)  I  empha- 
sized that  lyophilic  colloids  are  not  "only"  systems  of  the  composition  Liquid  + 
Liquid,  but  also  dispersoids  of  a  "higher  order,"  or,  in  the  words  used  above,  complex 
dispersoids.    This  has  not  been  taken  into  account  by  those  writers  who  have  objected 
to  my  characterizations  by  pointing  out  that  colloid  mercury  which  consists  of  two 
fluid  phases  has  no  lyophilic  properties.    They  have  erroneously  ascribed  to  me  the 


view  that  all  Liquid  +  Liquid  systems  have  such  properties,  while  actually  I  merelv 

11-1.1  •*.*.  »  i  *i*  ii*i«i.*  r 

ims.     But  it  may 


view  tnat  all  .Liquid  -f-  Liquid  systems  nave  sucn  properties,  wnile  a< 
held  the  narrower  opinion  that  lyophilic  colloids  belong  to  these  systei 
be  pointed  out  again  that  for  the  reasons  given  on  p.  47,  complex  systems  may  be 
expected  in  systems  of  the  type  Liquid  +  Liquid  with  greater  certainty  than  in  those 
of  the  type  Liquid  +  Solid.  A  complex  composition  is  therefore  more  general 
and  commoner  in  a  Liquid  +  Liquid  dispersoid.  If  colloids  of  the  composition 
Liquid  +  Liquid  were  unknown  it  would  be  necessary  to  seek  them  and  they  would 
no  doubt  be  easily  found. 


54  GENERAL  COLLOID-CHEMISTRY 

lated  but  be  simplified  and  explained  if  the  differences  in  the  types  of 
the  disperse  phases  and  the  consequences  thereof  are  kept  in  mind. 

5.  Relation  of  These  Two  Colloid  Classes  to  Molecular  Dis- 
persoids. — Many  investigators   have  pointed   out   that   greater 
similarities  exist  between  "lyophilic"   colloids  and  typical  mo- 
lecular dispersoids  than  between  the  latter  and  colloids  of  the 
type  Liquid  +  Solid.     Without  going  into  details  which  must  be 
reserved  for  later,  we  may  emphasize  that  such  relations  may  be 
expected    on    the  mere    basis   of    our   characterization    of    the 
"lyophilic"    colloids    as    Liquid  +  Liquid    systems.      Physical 
chemists   have   recently   become   increasingly   certain   that   the 
highly  disperse  phases  of  molecular  and  supermolecular  solutions 
must  be  conceived  of  as  combined  with  a  number,  sometimes  a 
very  considerable  number  (100  and  more),  of  the  molecules  of 
the  solvent  as  sohates.     Even  though,  as  emphasized  before,  we 
cannot  speak  of  the  state  of  aggregation  of  a  molecule,  such  a 
union  of  solvent  with  molecule  cannot  be  conceived  of  physically 
other  than  as  a  highly  disperse  liquid.     As  a  matter  of  fact,  re- 
cent workers   on   the  theory  of  solution  speak  of  "  droplets."1 
This  widespread  impression  of  the  existence  of  a  closer  relation- 
ship between   "true   solutions"   and   "lyophilic"   colloids   than 
between  the  former  and  "lyophobic"  systems  therefore  corre- 
sponds with  the  conceptions  above  presented. 

6.  Suspensoids  and  Emulsoids. — In  view  of  the  relations  exist- 
ing between  these  two  classes  of  colloids  and  the  corresponding 
coarse  dispersions  it  seems  expedient  to  give  the  former  special 
names.     R.    Hober2    introduced   the   name   suspension   colloids 
for  the  colloids  of  the  type  Liquid  +  Solid.     An  analogous  term 
for  the  second  class  would  be:  emulsion  colloids.     The  abbrevia- 
tions suspensoids  and  emulsoids  have  been  suggested  by  P.  P. 
von  Weimarn.3     These  will  be  employed  in  the  succeeding  pages 
of  this  book.     If  one  wishes  to  characterize  a  colloid  in  greater 
detail  one  may  speak  of  "poly suspensoids"  (systems  composed 
of  solid  phases  having  different  degrees  of  dispersion),  of  "com- 
plex emulsoids,"  etc.    No  objection  can,  of  course,  be  raised  against 
expressions  like  "lyophilic  emulsoids."     Only  it  should  be  re- 
membered that  the  terms  "suspensoids"   and  "emulsoids"  in 

1  See  K.  Drucker,  Zeitschr.  f.  physik.  Chem.,  67,  634  (1909). 

2  R.  Hober,  Physik.  Chem.  d.  Zelle.,  2  Aufl.,  208,  Leipzig,  1906. 

3  P.  P.  Von  Weimarn,  Koll.-Zeitschr.,  3,  26  (1908). 


GENERAL   PROPERTIES   OF    COLLOID    SYSTEMS  55 

contradistinction  to  "lyophilic"  and  "lyophobic  colloids"  have 
the  advantage  of  expressing  more  definite  and  hence  more  fruitful 
views  regarding  the  properties  of  the  dispersoids.  A  view  which 
connects  the  state  of  the  disperse  phase  with  the  general  concep- 
tion of  the  dispersoid  seems  incomparably  more  concrete,  more 
useful,  experimentally,  and  more  suggestive  than,  for  example, 
the  conception  of  "lyophilia." 

§11.  Transition  Phenomena  between  Suspensoids  and  Emulsoids 

As  already  mentioned,  it  is  possible  for  a  phase  to  pass 
smoothly  from  a  solid  to  a  liquid  state  and  vice  versa.  Often  such 
progressive  changes  may  occur  during  the  process  of  coagulation 
in  one  and  the  same  system,  as  in  a  complex  dispersoid.  Thus 
an  originally  liquid  disperse  phase  may  be  precipitated  in  an 
almost  solid  condition  by  appropriate  means  of  coagulation.  Such 
a  transition  from  emulsoid  to  suspensoid  demonstrates  par- 
ticularly well  the  properties  which  result  from  a  change  in  the 
state  of  the  disperse  phase.  According  to  J.  Friedlander,1  for 
example,  two  kinds  of  systems  may  be  prepared  from  alcohol,  rosin 
and  water,  both  of  which  are  turbid,  thus  proving  them  disperse 
heterogeneous  systems.  The  first  of  these  is  made  by  pouring  a 
few  drops  of  an  alcoholic  solution  of  rosin  into  an  excess  of  water 
when  the  rosin,  which  is  practically  insoluble  in  water,  separates 
out  as  a  solid  disperse  phase  while  the  alcohol,  in  greater  part  at 
least,  is  dissolved  in  the  water.  The  second  is  made  by  adding 
a  few  drops  of  water  to  a  concentrated  alcoholic  solution  of  rosin. 
In  this  case  the  first  drops  of  water  probably  dissolve  in  the 
rosin-alcohol,  but  further  amounts  can  dissolve  only  in  the  alcohol 
or,  perhaps,  succeed  in  withdrawing  this  from  the  solution  so 
that  small  droplets  of  water-alcohol  (liquid)  appear  in  the  liquid, 
alcoholic  solution  of  rosin  and  make  it  turbid.  A  disperse  hetero- 
geneous system  with  a  solid  disperse  phase  as  well  as  one  with  a 
liquid  disperse  phase  may  therefore  be  prepared  from  the  same 
three  components  by  appropriate  changes  in  their  concentration. 
Friedlander  found  the  behavior  of  the  two  systems  to  be  entirely 
different.  "Such  a  turbid  mixture  (a  concentrated  alcoholic 
solution  of  rosin  to  which  a  little  water  has  been  added)  behaves 
very  differently  from  the  ordinary  rosin  suspension  in  that  it  is 

1  J.  Freidlander,  Zeitschr.  f.  physik.  Chem.,  38,  430  (1901). 


56  GENERAL  COLLOID-CHEMISTRY 

not  coagulated  by  an  increase  in  temperature  or  on  the  addition 
of  electrolytes.  When  the  temperature  is  lowered  the  rosin 
phase  becomes  solid  but  is  not  coagulated,  for  a  rise  in  temperature 
restores  the  system  to  its  previous  condition.  Although  pre- 
viously irreversible,  the  system  is  now  completely  reversible."1 
Friedlander  further  found  the  internal  friction  of  the  second  kind 
of  system  to  be  greater  than  that  of  the  first.  In  this  respect  the 
second  system  closely  resembles  typical  emulsions  such  as  those 
of  isobutyric  acid  in  water.  A  detailed  study,  qualitative  and 
quantitative,  of  these  systems  would  evidently  be  of  great  interest 
for  the  classification  and  characterization  of.  disperse  systems 
on  the  basis  of  the  state  of  the  disperse  phases  entering  into  their 
composition.  Transitions  from  suspensoids  to  emulsoids  and 
vice  versa  exist  also  among  the  colloids  proper.  Nearly  all 
protein  solutions,  for  example,  are  emulsoid  in  character;  they 
are  viscous,  flocculated  only  by  large  quantities  of  electrolytes, 
etc.  Yet  0.  Hammarsten,2  found  that  a  neutral  solution  of 
salt-free  serum  globulin  is  coagulated  by  minute  quantities  of 
salt  (o.i  to  0.3  per  cent.  Nad);  and  according  to  W.  Erb3  the 
same  is  true  of  a  plant  protein,  vitellin.  According  to  H.  Freund- 
lich  and  W.  Neumann,4  many  dyes  show  an  emulsoid  character 
in  aqueous  solutions  and  a  suspensoid  character  in  alcoholic 
solutions.  Solutions  of  these  substances  in  mixtures  of  the  two 
dispersion  means  must  evidently  exhibit  transitions  between 
suspensoids  and  emulsoids  similar  to  those  which  Fridlander 
discovered.  Systematic  investigations  in  this  field  would  also  be 
of  importance  for  the  theory  of  the  colloid  state.  Finally,  we 
will  here  point  out  that  one  and  the  same  substance  may  appear 
either  in  the  suspensoid  or  in  the  emulsoid  state  in  one  and  the 
same  dispersion  means  depending  only  upon  the  conditions  under 
which  it  is  prepared. 

§12.  The  Crystalline  (Vectorial)  Constitution  of  the 
Disperse  Phase 

i.  The  Concept  of  Crystallinity.  —  As  is  well  known,  most  solid 
substances  as  well  as  a  limited  number  of  liquids  are  character- 

1  J.  Freidlander,  I.e.,  432,  433. 

2  O.  Hammarsten,  Pfluger's  Arch.,  18,  38;  see  also  Zeitschr.  f.  physiol.  Chem.,  395 


3  W.  Erb,  Zeitschr.  f.  Biol.,  41,  i  (1901). 

4  H.  Freundlich  and  W.  Neumann,  Koll.-Zeitschr.,  3,  80  (1908). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  57 

ized  by  the  fact  that  when  their  viscosity  is  sufficiently  great 
their  optical,  elastic,  dielectric,  etc.,  properties  are  dependent 
upon  the  arrangement  of  their  molecules  in  space.  Besides  the 
vectorial  nature  of  these  and  other  properties  of  such  systems, 
which  we  usually  designate  as  crystalline,  these  systems  assume  a 
definite  external  shape  when  their  internal  friction  is  sufficiently 
great.  In  the  most  characteristic  cases  this  external  shape  is 
made  up  of  a  series  of  plane  surfaces.  A  detailed  discussion  of 
the  distribution  of  the  crystalline  state  in  nature,  or  of  the  question 
of  whether  so-called  amorphous  solids  are  only  under-cooled 
liquids1  is  out  of  place  here.  Let  it  be  noted,  however,  that 
some  investigators  like  M.  L.  Frankenheim2  and  P.  P.  von 
Weimarn  are  so  convinced  of  the  wide  distribution  of  crystal- 
linity  or  vectoriality  that  they  have  declared  the  crystalline  state 
"the  only  internal  state  of  matter."  P.  P.  von  Weimarn,  es- 
pecially, believes  that  the  crystalline  (vectorial)  state  is  char- 
acteristic of  all  solid,  liquid  and  even  gaseous  substances,  and 
that  generally  speaking  no  amorphous  substances  exist  in  nature.3 
But  evidently  there  has  been  confused  here  the  possibility  of 
demonstrating  crystalline  (vectorial)  properties  in  all  manner  of 
substances  in  every  state  with  the  actual  existence  of  vectoriality 
in  these  as  postulated  by  P.  P.  von  Weimarn.  While  all  gases 
may  be  transformed  into  liquids  and  most  of  these  into  crystalline 
solids,  only  a  relatively  small  number  of  liquids  (and  of  these 
only  certain  ones  which  exhibit  special  chemical  properties 
such  as  "molecular  chain  formation,"  etc.)  are  possessed  of 
experimentally  demonstrable  crystalline  properties  when  in  the 
liquid  state;  and  up  to  the  present  time  no  evidence  at  all 
is  at  hand  to  indicate  the  existence  of  a  crystalline  structure 
in  gases.  From  this  it  follows  that  the  "intensity  of  the  vec- 
torial chaining  together  of  the  molecules"  (P.  P.  von  Weimarn) 
is  so  slight  in  all  gaseous  and  most  liquid  systems  that  it  is 
of  no  importance.  The  assumption  of  vectoriality  in  these 
systems  is  in  consequence  superfluous,  for  it  leads  to  no  fruitful 
deductions. 

1  See  especially  the  recent  and  extensive  discussion  with  references  to  the  litera- 
ture by  C.  Doelter,  Koll.-Zeitschr.,  7,  29,  86  (1910). 

2  P.  P.  Von  Weimarn.     See  his  numerous  discussions  in  Koll.-Zeitschr.,  2,  and 
subsequent  volumes,  especially  6,  32  (1910). 

3  The  earlier  literature  is  extensively  discussed  and  in  part  cited  verbatim  by 
O.  Lehmann,  Molekularphysik,  I,  716,  Leipzig,  1888. 


58  GENERAL  COLLOID-CHEMISTRY 

It  must  further  be  emphasized  that  the  concept  of  crystal- 
linity  or  vectoriality  is  as  ambiguous  a  one  as  is  that  of  hetero- 
geneity (see  the  next  paragraph) .  For  a  system  may  be  vectorial  or 
crystalline  in  certain  of  its  properties  while  it  is  isotropic  in  others. 
All  solid  crystals,  for  example,  are  vectorial  in  shape,  but  crystal- 
line liquids  have,  generally  speaking,  only  an  optical  vectoriality. 
On  the  other  hand,  all  solid  crystals  of  the  regular  system,  for 
instance,  are  not  vectorial  in  their  refraction  coefficients.  Other 
types  of  crystals  exhibit  different  degrees  of  optical  vectoriality. 
A  characterization  of  systems  according  to  their  vectoriality  is 
therefore  somewhat  arbitrary,  since  it  is  always  necessary  to  state 
which  of  the  properties  are  vectorial.  The  failure  of  investigators 
to  consider  that  different  kinds  of  crystalline  systems  and  different 
kinds  and  degrees  of  vectoriality  must  be  distinguished  accord- 
ing to  the  kind  and  the  number  of  the  properties  of  the  vectorial 
state  has  undoubtedly  contributed  its  share  toward  confusing 
the  problem  of  the  relations  between  crystalline  and  amorphous, 
solid  and  liquid  states  of  substances. 

2.  Direct  Proof  of  Crystallinity  in  Colloids. — The  most  fre- 
quently applied   and  simplest  practical  method   of  recognizing 
crystalline    properties    is    the   optical.     As    indicated    in   their 
definition,  it  must  be  impossible  to  prove  by  any  direct  methods 
such  as  the  microscopic,  that  colloids  possess  a  crystalline  struc- 
ture.    Ultramicroscopic    methods    in    place    of  microscopic   can 
only  be  of  limited  use,  for  they  give  no  direct  "image"  of  the 
object.     A   whole    series    of    optical   facts   have,    nevertheless, 
been   accumulated    in    favor    of    a    crystalline    constitution  of 
the  disperse  phase  of  metallic  sols.     These  will  be  discussed  in 
detail  when  we  consider  the  optical  properties  of  colloid  sys- 
tems.    Upon     such     and     similar    grounds,    investigators    like 
R.   Zsigmondy,    H.    Siedentopf,    A.    Cotton    and    H.    Mouton 
have  been  led  to  believe  in  the  possibility  if  not  in  the  prob- 
ability of  the  crystalline  nature  of  metallic  sols  at  least. 

3.  Indirect  Proof  for  the  Crystallinity  of  Colloid  Phases. — The 
Crystallinity  Theory  of  P.  P.  von  Weimarn. — Since  we  have  no 
direct  evidence  besides  the  ultramicroscopic  upon  which  to  base 
conclusions  regarding  the  vectorial  state  of  colloid  disperse  phases 
we  are  compelled  to  resort  to  indirect  means  based  upon  theoretical 
considerations  and  exterpolations.     Most  of  these  conclusions 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS 


59 


are  based  upon  the  assumption  that  particles  retain  their  crys- 
tallinity  even  when  their  size  is  progressively  changed.  Such  con- 
clusions were  drawn  early  in  the  history  of  colloid-chemistry;  and 
if  the  "reguline"  state  of  a  metal  may  be  considered  as  crystal- 
line or  cryptocrystalline,  B.  J.  Richter  (1862)  may  be  regarded  as 
the  first  to  have  urged  the  view  that  suspensoid  phases  have  a 
crystalline  constitution.  By  far  the  most  convincing  evidence 
in  favor  of  the  view  that  the  disperse  phase  retains  its  crystalline 


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60  GENERAL  COLLOID-CHEMISTRY 

(b)  Another  point  in  favor  of  the  crystalline  constitution  of 
suspensoid  phases  is  their  power  of  starting  crystallization  in 
supersaturated  molecular-disperse  solutions  of  themselves.     Gener- 
ally speaking,  only  such  solids  have  this  power  which  are  them- 
selves crystalline.     Yet  as  von  Weimarn  himself  found,  highly 
disperse  sols  lose  this  power  when  their  degree  of  dispersion  is 
sufficiently  increased.     It  is  fair  to  attribute  this  to  the  law  that 
the  solubility  of  a  substance  is  dependent  upon  its  specific  surface, 
that  is  to  say,  rises  greatly  with  extreme  subdivision  (see  p.  74). 
Highly  dispersed  particles  would  therefore  not  initiate  crystalliza- 
tion in  supersaturated  molecular-disperse  solutions,  because  the 
latter  are  still  unsaturated  with  regard  to  them.     Wilhelm  Os  t- 
wald's    finding1    that    small    quantities  of    salol,  made    highly 
disperse  by  trituration  with  an  indifferent  substance,  are  unable  to 
effect  the  crystallization  of  supermolten  salol,  even  though  the 
salol  is  still  demonstrable  analytically,  may  also  be  thus  interpreted. 

(c)  That  the  particles  of  sols  may  coalesce  to  form  micro- 
crystalline  bodies  and  even  definite  crystals  after  long  standing  is 
another  fact  in  favor  of  the  crystalline ty  of  suspensoid  phases. 
Thus  P.  P.  von   Weimarn2  found  silver  crystals  to  form  in  a 
silver  hydrosol  after  this  stood  a  while.     The  ultramicroscopic 
observations  of  M.  Traube-Mengarini,3  of  J.  Amann4  and  of  L. 
Pelet  and  A.  Wild5  who  noted  the  direct  formation  of  crystalline 
bodies  by  a  simple  coalition  of  ultramicroscopic  particles  in  colloid 
lead  (lead  oxyhydrate),  colloid  iodine  and  colloid  dyes  are  even 
more  convincing  evidence  of  the  possibility  of  a  "direct  colloid 
crystallization,"    that   is,    a    direct   fusion    of   ultramicroscopic 
particles  to  form  definite  crystals.     One  is  inclined  to  believe  that 
only  vectorial  particles  can  have  the  power  of  growing  into  definite 
crystals,  just  as  one  believes  that  only  such  may  produce  crystalli- 
zation in  supersaturated  solutions.     Yet  we  must  point  out  even 
here  that  the  crystalline  character  of  these  "crystalline  elements " 
has  been  disputed  by  a  whole  series  of  investigators  (see  below) . 

From  these  and  other  facts  we  may  conclude  that  most  sus- 
pensoids,  that  is  dispersoids  having  a  degree  of  dispersion  of  6.1  o5 

1  Wilh.  Ostwald,  Z.  f.  physik.  Chem.,  22,  289  (1897). 

2  P.  P.  von  Weimarn,  Koll.-Zeitschr.,  4,  317  (1908);  5,  62  (1909). 

3  M.  Traube-Mengarini  and  A.  Scala,  Koll.-Zeitschr.,  6,  65  (1910). 

4  J.  Amann,  Koll.-Zeitschr.,  6,  235  (1910). 

5  L.  Pelet- Jolivet  and  A.  Wild,  Koll.-Zeitschr.,  3,  175  (1908). 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  6 1 

to  6.  io7  are  possessed  of  a  crystalline  disperse  phase.  But  serious 
objections  may  be  raised  to  the  assumption  that  all  solid  disperse 
particles  are  crystalline  as  P.  P.  von  Weimarn,  for  example,  has 
advocated.  Thus,  as  mentioned  above,  the  crystallinity  of  large 
masses  of  all  solids  under  all  circumstances  has  not  been  demon- 
strated experimentally.  Even  though  most  substances  may  be 
obtained  in  crystalline  form,  yet  under  many  circumstances  the 
"vectorial  chaining  together  of  the  molecules"  is  so  slight  or  so 
loose  that  vectorial  properties  are  no  longer  observable.  While  it 
is  true  that  proteins  may  be  obtained  in  crystalline  form,  yet  the 
solid  precipitates  from  protein  solutions,  except  as  produced  under 
special  conditions,  exhibit  no  trace  of  crystallinity.  Under  such 
circumstances  it  is  therefore  just  as  suitable  to  assume  that  the 
intensity  of  vectorial  chaining  is  zero  as  to  postulate  a  " latent" 
crystallinity. 

4.  Dependence  of  Crystallinity  upon  Size  of  Particles. — There 
remains  the  possibility  that  the  general  assumption  upon  which  all 
these  indirect  proofs  of  the  crystalline  nature  of  the  colloid 
disperse  phase  depend,  namely,  the  retention  of  vectoriality  in 
extremes  of  dispersion,  is  not  valid  for  the  degrees  of  dispersion 
here  under  discussion.  From  the  behavior  of  liquids  in  the  proc- 
ess of  solidification  we  are  compelled  to  assume  that  solids  have  a 
positive  surface  tension  even  though  its  effects  do  not  become 
clearly  evident  because  of  the  great  internal  friction  possessed  by 
solid  substances.  But,  as  will  be  discussed  later,  the  surface 
energy  expressive  of  this  surface  tension  increases  markedly  with 
every  increase  in  the  specific  surface;  in  other  words,  a  greater 
centripetal  force  acts  upon  the  molecules  of  highly  disperse 
particles  than  upon  those  of  coarsely  disperse  particles.  It  seems 
not  impossible  that  such  a  positive  surface  tension  may  produce  a 
deformation  in  minute  crystals,  in  other  words,  destroy  their 
structural  vectoriality  by  rounding  off  their  corners  and  trans- 
forming them  into  spheroidal  bodies.  As  shown  by  the  behavior 
of  liquid  crystals,  the  optical  vectoriality,  for  example,  of  such  a 
particle  need  not  be  destroyed  in  such  a  process.  It  could  there- 
fore be  possible  that  the  free  surface  tension  of  solid  particles 
might  attain  values  in  extreme  degrees  of  dispersion  sufficient  to 
destroy  the  vectorial  chaining  together  of  the  molecules  responsible 
for  crystallinity.  An  investigation  of  the  influence  of  pressure 


62  GENERAL  COLLOID-CHEMISTRY 

upon  the  optical  properties  of  crystalline  liquids  would  be  of  interest 
in  this  connection.  Further,  it  might  be  possible  that  a  relation 
exists  between  compressibility  and  vectoriality  of  such  a  nature 
that  easily  compressible  substances  lose  their  structural  vectoriality 
at  lower  degrees  of  dispersion  than  less  compressible  ones,  etc. 

It  is  of  importance  that  such  an  influence  of  the  free  surface 
tension  which  increases  with  the  specific  surface  is  not  only  con- 
ceivable theoretically  but  is  often  demonstrable  experimentally. 
In  fact  the  influence  of  this  factor  has  been  repeatedly  observed 
in  that  most  striking  expression  of  the  vectoriality  of  any  system, 
namely,  its  crystalline  form.  As  long  known  from  microscopic  ob- 
servation of  processes  of  crystallization,  small  spherical  bodies 
(globulites)  are  first  seen  to  appear  which  in  no  way  resemble 
crystals.1  It  is  only  after  these  globulites  have  attained  a  certain 
size  that  they  assume  crystalline  shape.  Crystals  with  rounded 
edges  are  seen  to  appear,  and  so  on.2  According  to  Link,  Franken- 
heim,  Vogelsang,  Behrens,  Quincke,  Biitschli,  and  many  others, 
crystals  are  of  ten  formed  by  the  coalescence  of  these  microscopically 
isotropic  globulites,  from  which  there  then  result  "margarites," 
"  honeycombs,"  etc.3  It  would  be  of  interest  to  determine  whether 
other  changes  in  the  vectoriality  of  these  primary  crystals,  more 
particularly  changes  in  their  optical  properties,  also  develop  as  do 
the  structural  properties4  or  whether  they  exist  from  the  first  in 
even  the  smallest  globulites.  Such  a  microscopic  investigation 
might  perhaps  be  extended  to  ultramicroscopic  refraction  studies 

1  See  Wilh.  Ostwald,  Lehrb.  d.  allg.  Chem.,  2  Aufl.  I,  1042. 

2  See  the  beautiful  microphotographs  of  P.  P.  von  Weimarn  in  Koll.-Zeitschr., 
2  (1908). 

3  Splendid  photographs  of  such  honeycomb  structures  of   cystalline  materials 
are  found  in  O.  Biitschli,  Untersuchungen  iiber  Structuren,  Leipzig,  1898.     See  also 
the  numerous,  convincing   observations  of    G.  Quincke  [Ann.  d.  Physik.  (4),  9,  i 
(1902)]  as  well  as  the  earlier  monographs  of  H.  Behrens,  Die  Kristalliten,  Kiel,  1874; 
H.  Vogelgesang,  Die  Kristalliten,  edited  by  F.  Zirkel,  Bonn,  1875.     A.  partial  reprint 
of  the  early  views  may  be  found  in  O.  Lehmann,  Molekularphysik,  i,  730,  Leipzig, 
1908.     Especial  reference  should  be  made  to  the  excellent  observations  on  Asaron  of 
C.  Schmidt  [Liebig's  Ann. ,53, 171  (1845)]  who  observed  a  perfectly  regular  coalescence 
of  four  droplets.     For  a  discussion  of  the  vectorial  arrangement  of  coarsely  dispersed 
particles  see  R.  Krulla  [Zeitschr.  f.  physik.  Chem.,  66,  126  (1909)].     Wilh.  Ostwald 
[Lehrb.  d.  allg.  Chem.,  2,  Aufl.  i,  1040,  Leipzig,  1903]  also  recognizes  the  possibility  of 
a  "discontinuous"  development  of   crystals  from  particles  which  were  originally 
spherical.     But  in  the  end,  the  question  of  the  state  of  these  "crystal  embryos"  is 
still  to  be  regarded  as  open  (see  p.  61,  in  the  text). 

4  It  is  important  to  note   that  we  may  not  apply  to  all  matter  a  vectoriality 
observed  photographically  in  the  finest  precipitates  of  some  solid  substances.     The 
degree  of  effect  of  positive  surface  tension  upon  form  depends  also  upon  the  internal 
friction,  etc.,  of  the  particles  and  this  varies  considerably  in  different  cases  as  evi- 
denced by  the  so-called  "liquid  crystals."     See  in  this  connection  the  work  of  P.  P. 
von  Weimarn  cited  in  the  next  footnote  as  well  as  the  text  on  p.  61. 


GENERAL   PROPERTIES    OF    COLLOID    SYSTEMS  6 


of  colloids.  If,  for  example,  vectorial  differences  in  the  refraction 
coefficients  of  many  crystals  continue  to  exist  when  they  become 
extremely  small,  then  the  same  should  be  true  of  the  corresponding 
refraction  discs.  An  investigation  of  other  properties  of  highly 
disperse  solid  particles,  such  as  the  thermal  and  electrical,  would 
also  be  important. 

Attention  should  here  be  redirected  to  the  conclusion  reached 
above,  that  solid  particles  become  more  and  more  like  liquids 
as  their  degree  of  dispersion  increases ;  and  to  the  converse  of  this 
which  P.  P.  von  Weimarn  among  others  has  assumed  to  be  the 
case.  It  therefore  seems  possible  theoretically  that  a  development 
of  crystals  may  take  place  in  that  the  "crystal  embryos"  are  at 
first  liquid  and  only  later  become  solid  as  they  enlarge,  either  be- 
cause of  a  "progressive  "  coalition  of  molecularly-dispersed particles 
or  through  a  discontinuous  union  of  submolecular  phases.  That 
such  seems  to  be  the  case  is  evidenced  by  the  investigations  of  the 
observers  mentioned  above.  Wilhelm  Ostwald  (/.£.),  in  dis- 
cussing the  analogous  process  of  crystal  formation  in  molten 
masses,  even  says :  ' '  The  precipitation  of  the  insoluble  from  liquids 
seems  always  to  occur  primarily1  in  the  form  of,  droplets,  that  is, 
in  the  state  of  an  under-cooled  liquid."  If  the  dispersion  in  such  a 
system  were  to  become  fixed  at  such,  or  more  correctly,  at  a  some- 
what earlier  moment,  a  highly  disperse  system  (among  which 
colloid  systems  would  be  found)  containing  a  liquid  phase  would 
result.  In  other  words,  at  the  beginning  of  crystallization  a 
structural  vectoriality  would  be  lacking.  Whether  an  optical 
vectoriality  exists  at  this  stage  remains  to  be  determined.  Finally, 
it  seems  safe  to  assume  that  the  form  of  development  of  crystals 
will  also  vary  with  the  nature  of  the  crystallizing  substance. 

It  is  evident  from  all  this  that  the  question  of  the  maintenance 
of  crystallinity,  in  other  words  the  question  of  a  complete  vectori- 
ality of  the  disperse  particles,  more  particularly  of  the  disperse 
particles  of  solids  in  high  degrees  of  dispersion,2  cannot  yet  be 
settled  with  entire  certainty. 

1  The  italics  are  mine.     This  view  is  also  held  by  G.  Quincke  (Ann.  d.  Physik.,  9, 
10,  etc.). 

2  P.  P.  von  Weimarn,  in  a  recent  paper  [Koll.-Zeitschr.,  6,  32  (1910)]  holds  that  an 
influence  of  the  degree  of  dispersion  upon  the  form  of  solid  particles  only  becomes 
effective  if  their  size  is  less  than  5/iju,  especially  in  the  case  of  slightly  soluble  and 
difficultly  fusible  materials.     The  basis  for  this  is  derived  from  a  "purely  kinetic 
viewpoint"  dependent  upon  kinetic  views  regarding  the  physics  of  the  various 
" degrees  of  orientation"  of  molecules  in  the  body  and  in  the  surface  layers  of  a 
crystal.     I  confess  that  to  me  this  argument  is  not  convincing. 


64  GENERAL  COLLOID-CHEMISTRY 

5.  Crystallinity  of  Emulsoids. — Since  only  a  relatively  small 
number  of  crystalline  liquids  are  known,  we  may  expect  to 
encounter  crystalline  emulsoid  phases  but  rarely.  In  fact,  while  a 
number  of  coarse  emulsions  having  a  crystalline  disperse  phase  are 
known1  not  a  single  example  of  a  crystalline  emulsoid  is  known. 
This  is  in  part  due  to  the  fact  that  it  is  rarely  possible  to  make 
out  optically  the  particles  of  a  disperse  phase  and  thus  to  investi- 
gate their  vectorial  properties,  because  of  the  slight  difference  of 
refraction  between  them  and  the  dispersion  means.  It  is  of  course 
not  impossible  that  future  investigators  may  demonstrate  the 
existence  of  dispersoids  having  a  crystalline  emulsoid  phase.  In 
this  connection  the  behavior  of  crystalline  liquids  when  near  their 
"clarification  point7'  should  be -borne  in  mind  (see  the  literature 
cited  in  the  accompanying  footnotes). 

It  must  further  be  remembered  that  all  degrees  or  grades 
of  vectoriality  may  be  demonstrated,  particularly  in  liquids.2 
Not  only  do  we  find  examples  of  different  degrees  of  structural, 
optical,  etc.,  vectoriality  among  liquid  crystals  and  crystalline 
liquids,  but  as  shown  by  O.  Lehmann,  many  different  external 
factors  may  influence  the  kind  and  the  degree  of  vectoriality. 
Pressure  and  traction,  the  "adsorptive  power"  of  solids,  magnetic 
influences,  changes  in  temperature  or  of  the  solvent,  the  presence 
of  other  substances,  etc.,  are  all  of  importance.  There  are  liquids 
which  assume  vectorial  properties  only  under  the  influence  of 
powerful  external  agencies.  Thus,  A.  Cotton  and  H.  Mouton3 
showed  that  certain  organic  liquids  of  high  molecular  weight 
become  doubly  refractive  in  a  strong  magnetic  field.  Similar 
facts  have  long  been  known  regarding  many  typical  emulsoids,4 
such  as  concentrated  gelatine  solutions  (jellies)  when  under  the 
influence  of  pressure  or  traction.  As  is  well  known,  all  the 
contractile  elements  of  living  substance  exhibit  double  refraction.5 
Here  we  deal  with  a  temporary  vectoriality  which  exists  only  when 
certain  systems  are  under  the  influence  of  transitorily  active 

1  See  the  numerous  examples  in  O.  Lehmann,  Fliissige  Kristalle,  Leipzig,  1906. 

2  See  the  lecture  of  O.  Lehmann,  Fliissige  Kristalle  und  die  Theorien  des  Lebens, 
29,  Leipzig,  1906. 

3  A.  Cotton  and  H.  Mouton,  Compt.  Rend.,  141,  317,  349,  etc.  (1905). 

4  See  the  numerous  examples  investigated  by  G.  Quincke,  Drude's  Annalen  d. 
Physik.,  7,  9,  10,  n,  12,  13,  25  (1902  to  1904). 

6  A  recent  comprehensive  presentation  of  these  relations  may  be  found  in  W. 
Engelmann,  Ber.  Berl.  Akad.  d.  Wiss.,  694  (1906). 


GENERAL  PROPERTIES   OP   COLLOID   SYSTEMS  65 

agencies;  or  which  is  produced  through  the  absorption1  of 
submicroscopic  aniso tropic  particles.2  These  systems  would 
therefore  be  classed  as  possessing  the  lowest  possible  grade  of 
vectoriality  both  with  regard  to  intensity  and  to  number  of  vectorial 
properties.  From  all  of  which  it  becomes  somewhat  arbitrary 
whether  we  will  follow  O.  Lehmann3  and  P.  P.  von  Weimarn4 
in  describing  such  systems  as  possessed  of  an  "artificial  vectori- 
ality" and  as  " liquid-crystalline,"  or  not. 

1  H.  Ambronn,  Ber.  d.  D.  Botan.  Ges.,  6,  229  (1888);  7,  in  (1889);  Koll.- 
Zeitschr.,  6,  222  (1910). 

2  Details  regarding  double  refraction  in  emulsoids  will  be  found  in  a  forthcoming 
chapter  on  the  Optical  Properties  of  Colloid  Systems. 

3  O.  Lehmann,  Verh.  d.  D.  physik.  Ges.,  10,  321  (1908);  10,  406  (1908). 

4  P.  P.  von.  Weimarn,  Koll.-Zeitschr.,  3,  166  (1908). 


CHAPTER  III 

GENERAL  ENERGETICS  OF  THE  DISPERSOIDS 
§13.  Surface  Energies 

1.  Forms  of  Energy  Characteristic  of  Dispersoids. — The  fore- 
going pages   have   dealt   with   the   general   and    topographical 
characterization  of  dispersoid  systems,  more  particularly  colloid 
systems.     It  is  our  next  problem  to  discuss  the  more  important 
forms  of  energy  which  play  a ,  role  in  these — for  like  all  physical 
systems,  dispersoids  exhibit  phenomena  which  are  attributable  to 
changes  in  their  thermal,  radiant,  electrical,  chemical,  etc.,  energies. 
Evidently,  physical  systems  may  be  classified  on  the  basis  of  the 
forms  of  energy  which  appear  most  frequently  or  most  prominently 
in  them.     Thus,  gases  are  best  characterized  by   the  behavior 
of   their   volume  energies,  while  electrical  phenomena   seem  to 
be  especially  characteristic    of  dilute  salt  solutions.     The  form 
of  energy  most  characteristic  of  the  dispersoids  is  directly  deduc- 
ible  from  their  definition.     A  development  of  much  surface  is  the 
fundamental  property  of  dispersoid  systems.     But  the  absolute 
value    of    this    surface   is   a   direct  measure    of   the    capacity 
factor  of  the  so-called  surface  energies.     One    therefore   antici- 
pates that  the  properties  of  these  and  of  closely   related   forms 
of    energy  must   play   an    important   part   in   the    dispersoids. 
Especially  true  is  this  of  all  changes  in  the  dispersoid  state  which 
involve  an  increase  or  a  decrease  in  the  degree  of  dispersion; 
for  according  to  definition  every  change  in  the  magnitude  of  the 
surface  must  be  regarded  as  the  result  of  free  surface  energies  or  of 
their  compensation  by  other  energies.     Wilhelm  Ostwald  pointed 
out  the  importance  of  the  surface  energies    for    the   theory   of 
colloid  phenomena  even  before    their  dispersoid  character  was 
established  on  theoretical  and  experimental  grounds. 

2.  Surface  Energy  of  the  First  Order. — Surface  energy  as 
usually  discussed  is  made  up  of  two  components :  a  capacity  factor 
as  measured  by  the  absolute  surface,  and  an  intensity  factor  as 
measured  by  surface  tension.      This  type  of   surface  energy  en- 

66 


GENERAL   ENERGETICS    OF   THE    DISPERSOIDS  67 

deavors  to  decrease  the  surface  of  a  system  if  free  energy  is 
available.  For  reasons  to  be  discussed  in  the  succeeding  para- 
graphs we  shall  call  this,  surface  energy  of  the  first  order  and  its 
intensity  factor,  positive  surface  tension.  Its  most  important 
properties  are  the  following. 

If  surface  energy  of  the  first  order  is  freed  in  any  way  it  is 
changed  into  other  forms  of  energy,  especially  heat,  the  surface  of 
the  system  decreasing  at  the  same  time.  Conversely,  if  heat  is 
introduced  into  a  system  capable  of  developing  free  surface  energy 
of  this  order,  the  surface  tension  is  decreased.  Roughly,  the 
decrease  in  surface  tension  is  proportional  to  the  increase  in  tem- 
perature. If  an  electric  surface  is  produced,  in  that  two  phases 
having  different  electric  charges  which  are  not  permitted  to 
neutralize  each  other  are  brought  in  contact  with  each  other, 
the  surface  tension  of  the  phases  decreases.  Further,  the  value 
of  the  surface  tension  varies  with  the  chemical  character  of  the 
phases  which  are  in  contact  with  each  other.  General  laws  re- 
garding the  relation  between  magnitude  of  surface  tension  and 
chemical  character  of  the  phases  have  not  yet  been  discovered. 
The  surface  tension  of  a  dispersion  means  may  be  lowered  or  raised 
by  the  molecular-disperse  or  colloid  subdivision  of  a  phase  in  it 
(for  details  see  page  140).  The  value  of  the  total  surface  tension 
of  dispersoids  is  dependent  upon  the  age  of  the  surface.  If  the 
disperse  phase  lowers  the  surface  tension  of  the  dispersion  means, 
the  value  of  the  tension  decreases  with  time;  but  if  the  disperse 
phase  increases  the  surface  tension  of  the  dispersion  means,  little 
or  no  change  is  observable.  The  ultimate  value  of  the  surface 
tension  attained  after  a  longer  period  of  time  is  called  the  static 
surface  tension,  in  contradistinction  to  the  dynamic  surface  ten- 
sion observable  in  freshly  produced  systems.  We  shall  discuss 
the  reasons  for  such  changes  later.  Details  regarding  positive 
surface  tension  and  the  many  methods  of  measuring  it  with  its 
correlated  surface  energy  of  the  first  order  must  be  sought  in 
text-books  of  physics  and  physical  chemistry.1 

3.  Surface  Energy  of  the  Second  Order. — For  reasons  which 
we  are  unable  to  discuss  in  detail  here  we  are  compelled  to  recog- 
nize the  possibility  of  the  existence  of  another  form  of  surface 

1  A  recent  and  in  part  exhaustive  presentation  of  the  relation  of  positive  surface 
tension  to  other  physical  and  chemical  factors  may  be  found  in  H.  Freundh'ch,  Kapil- 
larchemie,  Leipzig,  1909. 


68  GENERAL  COLLOID-CHEMISTRY 

energy,  namely,  surface  energy  oj  the  second  order.  As  is  well- 
known,  two  forms  of  volume  energy  are  characteristic  of  gases:  one 
which  is  transformed  into  other  varieties  of  energy  when  the  volume 
of  the  gas  increases,  and  a  second  which  is  analogous  to  surface 
energy  of  the  first  order  in  that  it  also  is  converted  into  other  forms 
of  energy  when  the  volume  of  the  gas  decreases.  The  intensity 
factor  of  this  second,  less  well-known  form  of  volume  energy  is 
the  so-called  " internal  pressure."  In  liquids  this  attains  a  value 
estimated  at  several  thousand  atmospheres.  Reasoning  by  anal- 
ogy we  may  suspect  that  a  form  of  surface  energy  exists  which  has 
the  tendency  to  change  itself  into  other  forms  of  energy  whenever 
the  surface  of  a  system  increases.  The  intensity  factor  of  this 
type  of  surface  energy  might  be  designated  expansive  or  negative 
surface  tension.  What  evidence  is  there  for  the  actual  existence 
of  such  a  second  type  and  are  we  familiar  with  phenomena  which 
may  advantageously  be  explained  through  its  properties?1 

As  a  matter  of  fact,  certain  phenomena  are  known  which 
can  only  be  explained  by  assuming  the  existence  of  such  a  surface 
energy  of  the  second  order — an  expansive  surface  tension.  These 
are  the  increases  in  surface  which  occur  in  strictly  diphasic 
systems. 

The  simplest  and  clearest  expressions  of  an  expansive  surface 
tension  are  observed  when  small  volumes  of  liquid,  such  as  drop- 
lets or  streamlets,  are  electrified.  The  phenomena  have  long 
been  known  under  the  names  "  electric  heart,'7  "  electric  fountain/' 
etc.2  The  •  accompanying  Fig.  9  taken  from  O.  Lehmann  illus- 

1  We  frequently  encounter  in  the  literature,  as  in  the  writings  of  Maxwell,  Mens- 
brugghe,  Wilh.  Ostwald,  Fuchs,  van't  Hoff-Donnan,  M.  Heidenhain,  J.  Perrin,  L. 
Michaelis,  F.  Haber,  etc.,  discussions  of  the  possible  existence  of,  and  of  the  effects 
of  the  intensity  factor  of  this  kind  of  expansive  surface  tension.     Since  the  begin- 
ning of  1905,  partly  without  the  knowledge  of  the  studies  of  these  authors  and 
partly  before  their  papers  appeared,  I  have,  occupied  myself  with  this  concept  of 
surface  energy  of  the  second  order.     Since  it  led  to  conclusions  which  were  somewhat 
surprising  and  far  reaching,  I  did  not  dare  to  publish  a  monograph  entitled  "Unter- 
suchungen  zur  Theorie  der  Oberflachen- und  Volumenenergien"  even  though  the 
manuscript  had  been  revised  for  the  third  time  by  the  summer  of  1905.     It  has  been 
revised  and  enlarged  several  times  since  then  and  its  contents  subjected  to  rigid 
reexamination.      Because  similar  views  have  been  frequently  expressed,  and  encour- 
aged by  scientific  friends,  I  have  at  last  decided  to  publish  these  investigations,  even 
though  far  from  complete,  under  the  title,  "Die  energetische  Atomistik.  Unter- 
suchungen  zur  Theorie  der  Oberflachen-  und  Volumenenergien"  (Theodor  Stein- 
kopff,  Verlag,  Dresden).     Further  details  regarding  the  properties  of  surface  energy 
of  the  second  order  and  its  r61e  in  dispersoid  systems  may  be  found  there. 

2  Regarding    phenomena   of  this  type  see  O.  Lehmann,   Molekula  rphysik,  I, 
824,  Leipzig,  1888;  H.  Freundlich,  Kapillarchemie  212,  255,  260,  Leipzig,  1909.     [I 
do  not,  of  course,  agree  with  the  theories  of  the  latter  which  differ  fundamentally 
from  mine;  see  Wo.  Ostwald,  Koll.-Zeitschr.,  7,  142  (1910)]. 


GENERAL   ENERGETICS    OF   THE    DISPERSOIDS 


69 


trates  the  "disruptive"  surface  increase  against  turpentine  which 
liquid  (molten)  sulphur  shows  when  electrified.  The  left-hand 
figure  shows  the  effect  of  a  weak,  the  right-hand  that  of  a  strong 
charge.  The  liquid  sulphur  surrounding  the  rod-shaped  electrode 
first  assumes  conical  shape  at  the  tip  of  the  electrode  (this  already 
means  increase  of  surface)  and  then  breaks  up  into  individual 
droplets.  Through  strong  electrification  several  such  "points  of 
discharge"  all  showing  the  same  behavior,  may  be  produced. 
When  the  electrode  is  placed  in  a  vertical  position,  the  charge  is  high 


FIG.  9. — Increase  in  surface,  when  electrically  charged,  of  melted  sulphur  against 

turpentine  oil.     (After  0.  Lehmann.} 

The  left-hand  figure  shows  the  effect  of  a  weak,  the  right-hand,  the  effect  of  a 
stronger  charge. 

and  the  liquid  has  little  viscosity,  the  phenomenon  of  the  "electric 
fountain"  is  produced  (see  the  figure  in  O.  Lehmann's  volume). 
The  "electric  heart"  is  the  name  applied  to  the  changes  in  form 
observed  when  the  volume  of  liquid  is  weakly  electrified  (see  the 
point  of  the  electrode  in  the  figure  to  the  left). 

Such  increases  in  surface  have  also  been  observed  when 
solid  phases  are  brought  in  contact  with  liquid  ones  even  when  no 
electric  energy  is  available.  Of  recent  investigations  of  this  prob- 
lem those  of  M.  Traube-Mengarini,  A.  Scala,1  and  J.  Amann2 

1  M.  Traube-Mengarini  and  A.  Scala,  Koll.-Zeitschr.,  6,  65  (1910). 

2  J.  Amann,  Koll.-Zeitschr.,  6,  235  (1910).    For  other  examples  see  the  paragraphs 
on  direct  colloid  solution  in  Part  III. 


70  GENERAL  COLLOID-CHEMISTRY 

deserve  special  mention.  These  authors  were  able  to  observe 
microscopically  and  ultramicroscopically  the  breaking  up  of 
coarsely  disperse  particles  of  lead  or  iodine,  in  a  suitable  medium, 
into  smaller  but  not  amicroscopic  (molecular-disperse)  particles. 
J.  J.  von  Kossonogow1  found  that  electrification  promoted  these 
effects.  Another  striking  illustration  of  an  increase  in  the  surface 
of  a  "solid"  phase  is  seen  in  the  production  of  lead  sponge  from 
lead  plates  when  a  suitable  current  is  passed  through  them  (see 
pp.  71  and  82). 

In  complex  dispersoids  the  phenomena  characteristic  of  an 
expansive  increase  in  surface  remain  essentially  the  same,  but 
they  are  complicated  through  the  simultaneously  occurring 
changes  in  concentration  and  secondary  chemical  effects.  Expan- 
sion phenomena  are  observed  when  fatty  acids  come  in  contact 
with  alkaline  solutions;  when  cholesterin,  etc.,  come  in  contact 
with  various  pure  solvents,  etc.  The  so-called  "myelin  forms" 
produced  under  such  conditions  will  be  discussed  later. 

If  we  bear  in  mind  that  all  possible  transitions  exist  between 
coarsely  disperse,  colloid,  and  molecular-disperse  solutions,  we 
are  driven  to  the  ultimate  and,  perhaps,  most  important  conclusion 
of  all,  namely,  that  the  process  of  molecular  or  "true"  solution  is 
also  to  be  regarded  merely  as  such  a  spontaneous  and  extreme 
increase  in  surface  in  a  diphasic  system.2 

1J.  J.  Kossonogow,  Koll.-Zeitschr.,  7,  129  (1910)  where  earlier  publications  are 
listed. 

2  Even  the  most  modern  textbooks  of  physics  state  that  the  only  physical  require- 
ment for  solution  resides  in  a  reduction  of  the  positive  surface  tension  to  zero.  But 
this  really  tells  us  nothing  concerning  the  character  of  solution,  for  to  prove  the 
absence  of  an  energy  potential  gives  no  clue  to  the  source  of  the  work  necessary  for 
solution.  Especial  emphasis,  therefore,  must  be  laid  on  the  experimental  proof  of  a 
spontaneous  increase  in  surface  in  two-pha.se  systems.  Surface  increases  due  to  three 
positive  surface  tensions  have  long  been  noted  in  three-phase  systems  (as  in  the 
spreading  of  oil  on  water).  Regarding  the  view  that  solution  is  a  chemical  process 
consisting  of  the  formation  of  compounds  of  solvent  and  solute  in  indefinite  propor- 
tions, we  need  only  remark  that  this  assumption,  even  if  correct,  does  not  explain  the 
extraordinary  increase  in  surface  which  occurs  in  the  process  of  solution.  But  this 
increase  in  surface  is  by  definition  a  physical  process  which  like  all  other  physical 
phenomena  depends  among  others  upon  the  chemical  properties  of  both  phases  but 
also  upon  their  electrical,  thermal,  etc.,  properties,  all  of  which  influence  the  extent  of 
the  surface  increase.  No  chemical  conception  of  the  process  of  solution,  whatever  its 
nature,  is  able  to  explain  why  a  given  solid  (say  tannin)  dissolves  as  a  colloid  in 
one  solvent  (water)  and  as  a  molecular  dispersoid  in  another  (alcohol).  If  we  regard 
the  extensive  "division"  of  a  dissolved  substance  as  the  characteristic  of  both  colloid 
and  molecular-dispersoid  solution  then  every  process  of  solution  becomes  physical. 
We  can  only  speak  of  "chemical"  solution  (with  the  exceptions  noted  above)  when 
free  surface  energy  of  the  second  order  is  derived  exclusively  or  mainly  from  chemical 
energy.  The  solution  of  metals  in  acids  is  an  example  of  this  sort.  For  details  see 
the  book  announced  on  p.  68. 


GENERAL  ENERGETICS    OF   THE  DISPERSOIDS  71 

Regarding  the  remarkable  fact  that  separate  particles  are 
formed  immediately  in  the  expansive  increase  of  surface  in  the 
case  of  solid  phases  (with  the  exception  of  lead  sponge)  while  a 
progressive  increase  is  often  observed  in  liquids,  and  for  further  de- 
tails regarding  the  conditions  for  molecular  subdivision,  see  p.  77. 

4.  The  Relation  of  Surface  Energy  of  the  Second  Order  to 
Other  Forms  of  Energy. — Since  the  concept  of  expansive 
surface  energy  is  an  unfamiliar  one,  it  is  necessary  to  discuss 
briefly  its  relation  to  other  physical  and  chemical  factors.  Theo- 
retically, many  properties  of  this  surface  energy  of  the  second 
order  may  be  predicted,  and  this  on  the  basis  of  the  fact  that 
the  two  types  of  volume  energy  show  in  most  respects  a  reciprocal 
behavior.  Thus,  positive  surface  tension  decreases  as  a  rule  with 
increasing  temperature;  conversely,  expansive  surface  tension 
should  increase  when  the  temperature  increases.  This  require- 
ment is  satisfied  by  the  general  increase  in  solubility  which  sub- 
stances show  with  rising  temperature.  Lead  forms  spontaneously 
a  colloid  solution  in  distilled  water  at  room  temperature,  while 
silver  and  platinum  do  so  appreciably  only  when  boiled  (M. 
Traube-Mengarini  and  A.  Scala,  I.e.).  Further,  the  positive 
surface  tension  of  a  system  falls  if  a  difference  of  potential  is 
established  at  its  surface;  the  negative  surface  tension  should 
increase  under  such  circumstances.  That  such  is  the  case  was 
repeatedly  demonstrated  in  the  earlier  paragraphs  of  this  book. 
An  increase  in  surface  may  be  effected  very  generally  and  often 
strikingly  by  different  electrical  means,  as  in  the  production  of 
colloid  solutions  from  non-disperse  phases1  (electric  synthesis 
of  the  colloids) .  As  already  mentioned,  but  few  quantitative  re- 
lationships have  been  established  between  the  surface  tensions  of 
different  substances.  A  similarly  great  variation  should  therefore 
exist  in  the  values  of  the  expansive  surface  tension.  This  re- 
quirement is  satisfied  in  our  lack  of  stoichiometrical  generaliza- 
tions regarding  both  the  molecular-disperse  and  the  colloid 
solubility  of  substances,  etc. 

These  remarks  may  suffice  to  demonstrate  the  justice  of  assum- 
ing the  existence  of  a  surface  energy  of  the  second  order  with  the 
described  properties.  We  shall  accordingly  make  use  of  this 
concept  in  the  special  parts  of  this  book. 

1  See  Wo.  Ostwald,  Koll.-Zeitschr.,  7,  132  (1910). 


GENERAL  COLLOID-CHEMISTRY 


§14.  Dependence  of  Surface  Energies  upon  Specific  Surface 

i.  General  Considerations. — Relations  exist  between  the 
surface  energies  and  the  shape  of  the  phases  at  the  boundaries  of 
contact.  These  are  extremely  important.  First,  as  regards 
surface  energy  of  the  first  order:  As  is  well  known,  its 
most  striking  effects  appear  in  systems  which  have  markedly 
curved  surfaces  or  which,  when  possessed  of  plane  boundaries 
enclose  a  relatively  small  volume.  The  so-called  capillary 
phenomena  in  the  strict  sense  of  the  word  illustrate  the 
influence  of  the  markedly  curved  surface.  The  effect  of  the 
second  factor  is  illustrated  in  the  relation  which  exists  between  the 


FIG.  10. — Capillary  rise  and  specific  surface. 

height  to  which  a  liquid  ascends  between  two  glass  plates  which  are 
in  contact  with  each  other  along  one  edge,  and  the  thickness  of 
the  layer  of  the  liquid  and  of  the  gas  above  it  (see  Fig.  10) . 
The  thinner  the  layer,  the  more  definite  the  capillary  phenomena, 
that  is,  the  higher  the  ascent  of  the  liquid.  The  general  effect  of 
the  influence  of  the  curvature  as  well  as  of  the  thickness  of  the  layer 
of  the  liquid  upon  the  magnitude  of  the  surface  energy  of  the  first 
order  is  expressed  in  the  relation  between  the  surface  energy  and 
the  specific  surface  of  the  phases.  Thin  or  markedly  curved  layers 
of  a  liquid  are  manifestly  possessed  of  a  relatively  greater  absolute 
surface  than  equivalent  volumes  of  thicker  or  less  markedly  curved 
layers.1  An  increase  in  surface  energy  in  any  given  volume  is 

1  The    same  is   true   of    structures  in    which  two  dimensions  are  very  small 
(threads,  etc.). 


GENERAL   ENERGETICS    OF    THE   DISPERSOIDS  73 

therefore  produced  whenever  more  absolute  surface  is  devel- 
oped or  the  specific  surface  is  increased. 

When  we  apply  this  conclusion  to  typical  dispersoids  we  find 
that  a  given  volume  of  the  disperse  phase,  absolutely  considered, 
contains  more  surface  energy  than  the  same  volume  of  the  same 
substance  in  a  non-disperse  state.  But  the  total  amount  of 
surface  energy  of  a  single  particle  is  also  relatively  increased. 
Thus,  when  volume  and  mass  are  decreased  to  Kooo  by  decimal 
subdivision  of  a  cube  (see  Table  i),  the  surface  of  one  of  the  re- 
sulting cubic  particles  is  only  decreased  Hoo-  The  greater  the 
degree  of  dispersion,  the  more  surface  does  the  disperse  phase 
"contain.7'  In  fact  we  may  say  that  when  the  disperse  phase 
is  so  finely  subdivided  that  the  diameter  of  the  individual  par- 
ticles is  only  twice  that  of  the  sphere  of  action  of  molecular  forces, 
it  "consists  only  of  surface."  Evidently  the  shifting  in  any  sys- 
tem of  the  relation  of  the  different  kinds  of  energy  to  each  other 
in  favor  of  those  found  in  surfaces  must  have  a  fundamental 
influence  upon  the  character  of  these  systems. 

This  growth  of  the  surface  energies  with  increasing  subdivision, 
and  their  extraordinarily  great  importance  in  dispersoids  having  a 
high  specific  surface  may  be  further  illustrated  as  follows:1  If 
the  "internal  energy/'  that  is,  the  total  energy  of  a  system  minus 
the  surface  energy  is  designated  by  /,  and  the  surface  energy  by  S, 
then  the  total  energy  of  the  system  equals  I  -{-  S.  The  quantities 
of  energy  comprised  under  the  name  "internal  energy"  (for 
example,  kinetic  energy,  chemical  energy,  etc.),  are  proportional  to 
the  volume  v,  while  the  surface  energies  are  proportional  to  the 
surface  s,  in  other  words,  /  =  iv  and  S  =  ts  when  i  is  the  internal 
energy  of  the  unit  of  volume  and  t  is  its  surface  tension.  The  total 
energy  T  of  a  system  is  therefore  T  =  iv  +  ts.  If  now  we  consider 

T 
the  total  energy  of  the  unit  volume       =  Tv,  in  other  words,  if  we 

v 

is  \ 

divide  the  entire  equation  by  v  we  obtain  TV  =  i  +( --./).     If 

\v  J 

is  small,  that  is,  if  the  specific  surface  of  the  system  is  small,  the 

second  member  is  also  small  and  may  be  neglected.     This  is  the 
case  in  most  of  the  physico-chemical  reactions  hitherto'  investi- 

1  Wilh.  Ostwald,  Grundr.d.  allg.  Chem.,  4  Aufl.,  531,  Leipzig,  1909.    The  above 
is  a  somewhat  modified  presentation  of  the  subject. 


74  GENERAL  COLLOID-CHEMISTRY 

gated  in  which  interest  has  chiefly  centered  upon  part  i  of  the  total 
energy.  But 'if  v  is  kept  constant  and  5  is  increased,  as  in  the 
subdivision  of  a  given  cube  for  example,  the  second  member  may 
grow  tremendously  in  value.  If  the  subdivision  is  very  great, 
part  i,  which  is  proportional  to  the  volume,  may  disappear  alto- 
gether in  comparison  with  the  value  of  the  second  member  which 
becomes  infinitely  great  when  v  =  s.  Under  such  circumstances  the 
total  energy  of  the  system  consists  almost  entirely  of  surface 
energy  and  all  its  activities  are  characterized  by  the  properties  of 
the  latter.1 

2.  Surface  Energy  of  the  First  Order  and  Specific  Surface.— 
Illustrations  of  the  relations  between  surface  energy  of  the  first 
order  and  specific  surface  were  given  above.     Another  example  of 
such  a  relation  is  the  fact  that  the  height  of  ascent  of  a  liquid  in  a 
capillary  tube  is  inversely  proportional  to  the  diameter  of  the  tube; 
in  other  words,  the  product  of  the  height  of  ascent  and  the  diameter 
of  the  tube  is  a  constant.     This  means  that  if  the  diameter  of  the 
capillary  tube  is  reduced  by  half,  the  height  of  ascent  of  the  liquid 
is  doubled,  and  if  the  former  is  decreased  to  one- tenth  its  value,  the 
magnitude  of  the  latter  becomes  ten  times  as  great.     If  we  write 
the  surface  energy  of  a  cube  with  an  edge  i  cm.  long  as  i,  the 
surface  energy  of  the  same  cube  colloidally  subdivided  (so  as  to 
have  cubes  with  io/*/*  edges)  amounts  to  a  million  when  we  assume 
that  the  surface  tension  remains  unchanged. 

3.  Surface  Energy  of  the  Second  Order  and  Specific  Surface.— 
Since  the  surface  energy  of  the  second  order  contains  the  absolute 
surface  of  a  system  as  its  capacity  factor,  we  would  imagine  that 
its  effects  should  also  increase  with  increase  in  curvature,  decrease 
in  thickness,  or  increase  in  degree  of  dispersion.     Is  there   any 
experimental  evidence  for  this?     It  is  found  in  what  is  called  the 
influence  of  the  size  of  the  particles  of  solid  substances  upon  their 
solubility.      As  Wilhelm  Ostwald,2  G.  Hulett,3  and  others  have 
shown,  substances  in  a  finely  dispersed  state,  as  produced  by  tritu- 
ration  for  example,  are  more  soluble  than  those  in  a  coarsely  dis- 
persed state.     Hulett  found  finely  triturated  mercury  oxide  to  be 
more  than  three  times  as  soluble  as  coarser  pieces.     The  solubility 

1  See  p.  96  for  the  interesting  conclusions  deducible  from  this  discussion. 

2  Wilh.  Ostwald,  Z.  f.  physik.  Chem.,  34,  496  (1900). 

3G.  Hulett,  Z.  f.  physik.    Chem.,   37,  385  (1901);  see  also  Hulett  and  Allen, 
Journ.  Am.  Chem.  Soc.,  24,  667  (1902). 


GENERAL  ENERGETICS   OF   THE   DISPERSOIDS  75 

of  a  highly  triturated  powder  as  determined  by  conductivity 
measurements  amounted  to  0.694  millimols  (150  mg.  per  liter). 
An  especially  interesting  series  of  experiments  of  this  kind  were 
carried  out  by  Stas  in  the  year  1870  regarding  the  solubility  of 
the  different  "precipitates"  of  silver  chloride.1  Stas  found  that, 
depending  upon  the  experimental  conditions  under  which  it  is 
obtained,  silver  chloride  assumes  the  forms:  i.  "gelatineux;  2. 
caseeux,  flocconeux;  3.  pulverulent;  4.  grenu,  ecailleux,  crystallin 
fondu;"  and  that  the  solubilities  of  these  modifications,  the  degree 
of  dispersion  of  which  undoubtedly  decreases  in  the  order  given 
below,  was  as  follows: 

1.  Flocculent  silver  chloride  0.0140  gram  per  liter  at  20°. 

2.  Powdered  silver  chloride  0.0060  gram  per  liter  at  17°. 

3.  Granular    silver  chloride  o.oooi  gram  per  liter  at  15°. 

4.  Granular    silver  chloride   0.03     gram  per  liter  at  100°. 

The  solubility  of  the  granular  preparation  had  to  be  measured  at 
1 00°  because  it  is  too  slight  at  room  temperature  to  be  determined 
analytically.  The  solubility  of  the  gelatinous  chloride  could  not 
be  determined  because  of  the  difficulty  of  separating  it  in  this  con- 
dition from  the  fluid  in  which  it  is  precipitated  and  on  account  of 
its  instability. 

A  still  older  observation  of  this  kind  was  made  by  Thomas 
Graham.2  Graham  found  that  silicic  acid  jellies  of  different 
concentrations  have  different  (maximum)  molecular  solubilities. 
Thus,  only  two  parts  of  the  silicic  acid  of  a  i  per  cent,  jelly  formed 
a  molecular-disperse  solution  in  10,000  parts  of  water,  only  one 
part  of  a  5  per  cent,  jelly,  and  even  less  of  the  more  highly  con- 
centrated jellies.  But  silicic  acid  is  a  typical  emulsion  colloid,  that 
is,  its  degree  of  dispersion  changes  with  variations  in  concentra- 
tion. Concentrated  jellies  are  presumably  less  disperse  than  the 
more  dilute  and  so  have  a  lower  molecular  solubility. 

In  harmony  with  the  above-sketched  conception  of  solution 
as  a  process  of  extreme  increase  in  surface  produced  by  a  free 
expansive  surface  energy,  it  is  evident  that  such  influence  must 
act  by  effecting  an  absolute  increase  in  surface  energy  by  increas- 
ing the  specific  surface.  Such  a  relationship  is  rendered  plausible 
by  the  fact  that  the  " artificial"  breaking  up  of  a  substance 

1  See  K.  Drucker,  Koll.-Zeitschr.,  4,  216  (1909). 

2  Thos.  Graham,  Journ.  Chem.  Soc.,  1864;  see  also  his  collected  papers,  p.  618. 


76  GENERAL  COLLOID-CHEMISTRY 

preparatory  for  solution  already  represents  surface  work  which 
is  later  saved  in  the  process  of  that  further  surface  increase  which 
we  call  "solution." 

An  interesting  observation  apparently  contradicts  this  concep- 
tion of  an  increase  in  the  surface  energy  of  the  second  order  of  a 
system  with  its  degree  of  dispersion.  According  to  the  concur- 
rent statements  of  R.  Zsigmondy,1  J.  Donau2  and  The  Sved- 
berg3  colloid  gold  is  only  slightly  amalgamated,  if  at  all,  by  mer- 
cury. But  this  is  really  a  question  of  solution  velocity,  not  of 
maximum  solubility.  Besides,  this  case  should  not  be  compared 
with  what  was  said  above,  for  in  the  amalgamation  of  colloid  gold 
by  mercury  we  are  dealing  with  a  triphasic  rather  than  a  diphasic 
system;  furthermore,  an  absolutely  necessary  preliminary  condition, 
namely,  contact  of  the  two  phases  is  absent.  This  must  first  be 
produced  by  shaking,  etc.,  and  is  presumably  hindered  by  the  fact 
that  surfaces,  especially  when  markedly  curved,  are  surrounded  by 
liquid  films  having  special  properties  such  as  great  tenacity,  etc., 
which  must  be  broken  before  direct  contact  of  the  phases  and  solu- 
tion may  take  place  (see  later). 

4.  Dependence  of  Surface  Tensions  upon  Specific  Surface.— 
Besides  this  influence  of  the  specific  surface  upon  the  absolute  and 
relative  amounts  of  the  surface  energies,  there  exists  another  be- 
tween the  latter  and  the  shape  of  phases  encountered  when  equiva- 
lent but  differently  constituted  surfaces  are  compared.  This 
relation  depends  upon  the  circumstance,  which  has  both  an  experi- 
mental and  a  theoretical  basis,  that  the  direct  effects  of  the  sur- 
face energies  extend  to  a  certain  depth  on  both  sides  of  the  mathe- 
matical surfaces  of  contact. 

In  curved  surfaces  such  subsurface  effects  of  the  surface 
energies  may  weaken  or  strengthen  these,  depending  upon  the 
convexity  or  the  concavity  of  the  curvature  as  well  as  upon  the 
nature  of  the  phase.  Thus,  in  a  surface  which  is  convexly  curved 
with  regard  to  one  of  the  phases  and  which  has  a  positive  surface 
tension,  the  subsurface  effects  may  strengthen  each  other  in  the 
"convex"  phase  while  they  weaken  each  other  in  the  "concave" 
phase.  Since  we  must  believe  that  these  subsurface  effects  are 
produced  by  the  surface  energies  or  that,  conversely,  the  latter  are 

1  R.  Zsigmondy,  Liebig's  Ann.,  301,  37  (1899). 

2  J.  Donau,  Monatshefte  f.  Chem.,  26,  525  (1905). 
8  The  Svedberg,  Koll.-Zeitschr.,  5,  323  (1909). 


GENERAL   ENERGETICS    OF    THE    DISPERSOIDS  77 

the  result  of  changes  in  the  constitution  of  one  phase  produced  by 
contact  with  another,  this  mutual  weakening  or  strengthening  of 
the  subsurface  effects  must  have  a  reciprocal  influence  upon  the 
surface  energies,  more  particularly  upon  their  intensity  factors, 
the  surface  tensions.  If  the  simultaneous  and  opposite  strength- 
enings and  weakenings  of  such  subsurface  effects  produced 
through  curvature  of  the  surface  do  not  completely  neutralize  each 
other,  the  surface  tension  of  one  and  the  same  surface  may  assume 
different  values,  depending  upon  its  curvature. 

Special  relationships  are  encountered  when  the  curvature  is  so 
great  or  when  the  particles  are  so  small  or  when  a  layer  of  one  of 
the  phases  is  so  thin  that  the  layers  in  which  the  effects  of  the  sur- 
face energies  still  manifest  themselves  come  very  close  to  each 
other  or  into  actual  contact.  As  is  demonstrable  through  molecu- 
lar physics1  and  on  thermodynamic  grounds,2  the  intensity 
factors  of  the  surface  energies  change  much  under  such  circum- 
stances. This  variableness  of  the  positive  surface  tension  in  sys- 
tems having  small  dimensions  has  been  demonstrated  experimen- 
tally by  the  work  of  Reynold  and  Rucker3  on  soap  films.  Since 
we  have  no  direct  method  of  measuring  negative  surface  tension  in 
systems  having  small  dimensions,  experimental  demonstration  of 
its  variableness  has  not  yet  been  possible.  For  its  indirect  de- 
termination molecular  dispersoids,  or  better,  ionic  dispersoids 
might  be  used.  Special  attention  might  be  directed  to  the  prop- 
erties of  very  dilute  or  extremely  ionized  solutions  of  electrolytes 
and  their  conductivity  or  viscosity  peculiarities,  and  these  might 
be  correlated  with  variations  in  expansive  surface  tension. 

§15.  Reciprocal  Effects  of  the  Two  Surface  Energies 

(Theory  of  Dispersion  and  Condensation) 

i.  General  Considerations.— Ordinarily,  only  progressive  varia- 
tions, that  is  to  say,  uninterrupted  increases  or  diminutions  in 
surface  are  considered  when  the  phenomena  of  surface  tension  are 

1  See  Lord  Rayleigh,  Phil.  Mag.  (5),  30,  475  (1890). 

2  W.  Gibbs,  Thermodynamische  Studien,  274,  Leipzig,  1892;  van  der  Waals  and 
Kohnstamm,  Lehrb.  d.  Thermodynamik.,  i,  207,  Leipzig,  1908. 

3  Reynold  and  Rucker,  Phil.  Trans.  Roy.  Soc.  London  (2),  171,  447  (1881);  174 
645  (1883);  177,  627  (1886);  184,  505  (1893).     See  also  P.  Drude,  Ann.  d.  Physik, 
(3),  43,  158   (1891);  Johanott,  Phil.  Mag.  (5),  47,  501(1899);  (6),  n,  746  (1906); 
Schiitt,  Ann.  d.  Physik.  (4),  13,  712  (1904),  etc.;  also  A.  Pockels,  Nature,  43,  437 
(1891);  Lord  Rayleigh,  Phil.  Mag.  (5),  48,  331  (1899). 


78  GENERAL  COLLOID-CHEMISTRY 

• 

discussed.  While  the  coalescence  of  liquid  droplets  when  they 
come  in  close  contact  with  each  other  is  usually  attributed  to  a  sur- 
face tension  effect,  such  processes  are  less  satisfactorily  explained 
on  such  a  basis  alone  than  is,  for  example,  the  contraction  of  a  soap 
film.  Conditions  when  droplets  are  in  " close"  contact  are  highly 
complex  in  character  (see  later).  An  analogous  difficulty  is 
encountered  when  a  progressive  increase  in  surface  gives  way  to 
droplet  formation.  We  have  before  us  here  the  general  problem: 
Under  what  conditions  does  a  progressive  variation  in  surface  be- 
come discontinuous  ?  It  is  evident  that  this  question  is  of  special 
importance  in  the  dynamics  of  the  dispersoids,  more  particularly 
in  that  of  the  colloids,  for  these  are  produced  either  by  increasing 
the  dispersion  of  slightly  disperse  or  non-disperse  systems,  or  by 
condensing  maximally  disperse  (for  example,  molecular)  systems. 
2.  Discontinuous  Increase  in  Surface. — The  simplest  case  of  a 
progressive  increase  in  surface  is  encountered  when  we  observe  the 


Q 


a-  b  c 

FIG.  ii.  —  Spontaneous  changes  in  shape  of  drops  on  a  plane  surface. 

form  of  different  sized  droplets  of  a  non-wetting  liquid  such  as 
mercury  resting  on  some  solid  support  like  a  glass  plate  (see 
Fig.  1  1)  .  The  smaller  the  droplet,  the  greater  its  relative  (specific) 
surface  and  the  more  completely  does  it  retain  a  spherical  form  (a). 
Larger  droplets  become  flattened  by  their  own  weight  (b),  in  other 
words,  they  increase  their  absolute  surfaces  spontaneously,  for  their 
smallest  possible  surface  would  also  be  spherical.  If  we  attempt  to 
enlarge  the  volume  of  the  droplet  on  the  plate  by  adding  more 
liquid  to  it,  the  droplet  becomes  progressively  flatter  until  at  a 
maximum  volume,  different  with  different  liquids,  it  breaks  up 
into  several  smaller  droplets.  Thus,  with  ordinary  materials  it  is 
not  possible  to  make  a  coherent  droplet,  that  is,  a  continuous 
layer  of  more  than  about  25  cc.  of  mercury  on  a  glass  or  porcelain 
surface.1  An  analogous  phenomenon  is  offered  in  the  well-known 
fact  that  cylindrical  deformation  of  a  given  volume  of  liquid  can- 
not be  produced  after  a  certain  maximum  value  has  been  attained, 

1  It  is  not  denied  that  thinner,  continuous  layers  of  mercury  might  be  prepared  by 
other  methods  or  by  using  very  pure  materials. 


GENERAL    ENERGETICS    OF    THE    DISPERSOIDS  79 

without  having  the  liquid  thread  break.  We  need  but  call  to  mind 
the  difficulties  which  must  be  overcome  in  the  preparation  and 
progressive  deformation  of  fine  mercury  threads  in  the  making  of 
thermometers.  Subdivision  of  the  droplet  of  mercury  may  be 
facilitated  by  increasing  its  absolute  (and  specific)  surface  "arti- 
ficially" through  the  introduction  of  energy  from  without  as  by 
pressing  upon  it  with  a  glass  plate  as  shown  in  Fig.  n  c.  This 
increase  in  surface,  which  is  entirely  analogous  to  that  produced 
through  gravity,  leads  to  a  dispersion  of  the  drop  into  droplets 
which  are  at  first  irregular  in  size,  but  which  approximate  the  spher- 
ical more  and  more  as  they  become  smaller. 

Analogous  phenomena  are  observed  when  a  drop  of  rancid 
oil  is  placed  upon  a  very  dilute  alkaline  solution  in  which  it  changes 
its  shape  " spontaneously"  and  finally  emulsifies  itself;  or  when, 
at  a  temperature  of  4o°C.,  a  crystal  of  cholesterin  is  introduced 
into  a  solution  of  bile  salts.1  As  soon  as  the  progressive  def  orma«- 
tion  associated  with  increase  in  surface  has  attained  a  certain 
value  it  becomes  discontinuous  and  the  process  called  "dispersion" 
begins.  This  may  also  be  clearly  observed  in  the  electric  dis- 
persion of  liquids.  We  need  but  recall  the  facts  illustrated  in 
Fig.  9.  A  weak  electric  charge  produces  only  a  deformation 
and  enlargement  of  surface,  which  when  a  stronger  charge  is 
given  becomes  discontinuous  and  so  gives  rise  to  droplet  forma- 
tion. That  a  progressive  increase  of  surface  may  also  take  place 
in  the  "spontaneous"  production  of  colloid  and  molecular  dis- 
persoids  is  indicated  by  the  appearance  in  them  of  "solution 
figures."2  One  can  also  easily  see  that  the  greater  the  positive 
surface  tension  of  a  drop  of  liquid  as  compared  with  that  of  the 
medium  in  which  it  is  placed,  the  more  easily  will  its  dispersion 
be  accomplishable.  Thus,  on  the  same  glass  or  porcelain  plate 
a  drop  of  water,  or  better  yet  a  drop  of  ether,  may  be  spread  into 
a  much  thinner  continuous  layer  than  a  drop  of  mercury.  The 
corresponding  surface  tensions  are:  ether,  16.5  (at  20°),  water, 
70.6  (at  20°),  mercury  436  (at  15°).  A  somewhat  simpler  method 
of  demonstrating  this  relation  between  the  discontinuous  en- 
largement of  a  surface  and  its  surface  tension  is  to  deform  liquids 
by  causing  them  to  flow  through  a  capillary  tip  (see  Fig.  12). 

1  H.  Schade,  Kolloidchem.  Beihefte,  I,  377  (1910). 

2  See  the  earlier  compilation  in  O.  Lehmann,  Molekularphysik.,  i,  481,  Leipzig, 
1888;  where  striking  illustrations  may  also  be  seen. 


8o 


GENERAL  COLLOID-CHEMISTRY 


L                                         J 

\ 


V 


While  ether  and  water  may  flow  through  such  a  tip  in  a  fine  stream 
(a);  mercury  passes  through  in  the  form  of  droplets  (b). 

It  must  further  be  pointed  out  that,  as  far  as  known,  all 
phenomena  of  dispersion  are  connected  with  movement  of  the 
resulting  disperse  particles.  It  is  evident  that  such  spatial 
rearrangements  of  the  disperse  particles,  in  other  words,  these 
" dispersion  movements"  are  to  be  separated  theoretically  from 
the  process  of  dispersion  itself,  in  other 
words,  the  increase  in  surface.  They  are  to 
be  considered  as  phenomena  secondary  to 
the  transformations  of  energy  which  pro- 
duce dispersion.  Regarding  the  more  inti- 
mate relationships  between  the  intensity  of 
these  movements  and  the  dispersive  forces, 
only  suppositions  may  be  made,  for  no  exact 
investigations  of  them  exist  at  present. 

It  is  of  interest  to  consider  the  fate  of  an 
excess  of  free  surface  energy  of  the  second 
order  when  the  presence  of  a  large  amount 
of  surface  energy  of  the  first  order  prevents 
its  transformation  into  an  increase  of  surface. 
It  seems  plausible  to  assume  that  such  energy 
may  then  react  upon  the  liquid  dispersion 
means  in  such  a  way  as  to  transform  its  ten- 
sion  into  a  Pressure  acting  upon  its  surface 

and  of  great  (6)  surface    layer  (see  page  01).     This  assumption  is  sus- 
tension     when     issuing         .       .         '  ..  .  .  ,-•          ,1 

from  a  vessel.  tamed    by   the   well-known    fact    that   the 

boundaries  of  a  liquid  surrounding  another 
phase  show  those  special  properties  which  have  given  rise  to  the 
conception  of  "liquid  films."  We  shall  often  refer  to  these 
(see  page  87). 

3.  Theory  of  Dispersion. — All  increases  in  surface  are  regarded 
in  this  volume  as  expressions  of  surface  energy  of  the  second 
order.  The  work  necessary  for  such  transformations  is  made 
up  of  the  product  of  the  magnitude  and  of  the  tension  of  the 
surface.  It  is  therefore  by  definition  surface  work.  From  this 
point  of  view  all  increases  in  surface,  whether  produced  through 
gravity,  through  pressure  or  compression,  or  by  any  other  means, 
in  other  words,  all  processes  of  trituration,  pulverization,  com- 


GENERAL  ENERGETICS    OF   THE   DISPERSOIDS  8l 

minution,  etc.,  are  only  expressions  of  this  surface  energy  of  the 
second  order  and  differ  from  each  other  only  in  the  nature  of  the 
sources  of  the  energy  employed  in  bringing  about  the  increase. 
As  previously  emphasized,  not  only  mechanical  energies  but  also 
heat  and  electrical  energies  may  be  transformed  into  surface 
energy  of  the  second  order,  and  by  this  means  lead  to  an  increase 
of  surface.  Since  the  increase  in  surface  is  always  the  same,  inde- 
pendently of  the  nature  of  the  energies  employed  to  bring  it  about, 
it  must  remain  the  characterizing  feature  of  these  phenomena. 
If  we  consider  one  of  the  simpler  effects  of  surface  energy 
of  the  second  order,  as  the  deformation  of  a  drop  of  liquid  by 
its  own  weight,  with  regard  to  its  possible  effect  upon  the  surface 
energy  of  the  first  order,  we  reach  the  important  conclusion  that 
the  decrease  in  the  free  surface  energy  of  the  second  order  when 
converted  into  an  equivalent  of  other  energies  through  the  in- 
crease in  surface,  increases  the  amount  of  surface  energy  of  the  first 
order  in  the  system,  for  the  quantity  of  surface  energy  of  the  first 
order  in  a  system  is  proportional  to  the  absolute  surface  when 
the  intensity  factor  of  tension  remains  constant.  If  tension  is 
constant — which  is  certainly  the  case  in  the  non-disperse  and 
coarsely  disperse  systems  to  be  first  considered — the  amount  of 
surface  energy  of  the  first  order  increases  with  every  decrease  of 
the  other  surface  energies.  This  is  true  for  example  when  the 
surface  of  a  liquid  drop  is  progressively  increased.  But  there  is 
no  reason  for  assuming  that  the  increase  in  surface  energy  of  the 
first  order  is  always  equivalent  to  the  decrease  of  expansive 
surface  energy.  Experience  shows  (see  the  above-mentioned 
examples)  that  when  certain  increases  in  surface  are  brought  about, 
the  increase  in  surface  energy  of  the  first  order  is  greater  than  the 
decrease  in  surface  energy  of  the  second  order,  for  as  soon  as  there 
exists  an  excess  of  contractile  surface  energy  the  surface  of  the 
given  volume  becomes  discontinuous.  The  equilibrium  between 
the  two  energies  which  is  "dynamically"  displaced  by  a  slight 
deformation  of  the  surface  is  destroyed  as  soon  as  the  amount  of 
surface  energy  of  the  first  order  produced,  more  than  compen- 
sates for  the  decrease  of  expansive  surface  energy.  Equilibrium 
will  not  be  reestablished  until  the  liberated  amount  of  contractile 
surface  energy  has  been  transformed  (into  heat  for  the  most  part), 
a  change  which  can  be  accomplished  only  by  an  accompanying 
6 


82  GENERAL  COLLOID-CHEMISTRY 

diminution  in  surface.  Since  the  expansile  tension  prevents  a 
diminution  of  the  volume  as  a  whole  this  tendency  toward  dim- 
inution can  only  be  satisfied  by  a  subdivision  of  the  volume  into 
smaller  parts,  for  then  only  can  both  requirements  be  fulfilled  at 
the  same  time,  on  the  one  hand  the  increase  in  absolute  surface  as 
demanded  by  the  expansile  tension,  on  the  other  the  decrease  in 
absolute  surface  as  demanded  by  the  contractile  tension.  Sub- 
division is  the  only  possible  result;  or  to  put  it  in  another  way, 
the  reciprocal  effects  of  these  surface  energies  must  lead  to 
subdivision. 

Dispersion,  or  the  conversion  of  a  progressive  increase  in  surface 
into  a  discontinuous  one  is  characterized  energetically  by  a  liberation 
of  positive  surface  energy  brought  about  by  an  excessive  development  of 
absolute  surface  through  the  effects  of  expansile  surface  energy.1 

4.  Consequences  of  the  Energetic  Theory  of  Dispersion. — If 
the  suggested  conception  of  dispersion  is  correct,  a  number  of 
deductions  therefrom  must  be  capable  of  practical  support. 

It  follows  from  what  was  said  that,  neglecting  certain  transi- 
tion phenomena,  dispersion  should  set  in  suddenly  as  soon  as  a 
definite  amount  of  deformation  has  been  induced,  for  discontinu- 
ous increase  in  surface  corresponds  to  an  intersection  point  of  two 
changes  in  energy.  We  should  expect  to  encounter  especially 
clear  examples  of  such  "critical"  points  when  increases  in  surface 
are  produced  by  the  transformation  of  other  energies  into  surface 
energy  of  the  second  order,  for  then  a  better  control  of  conditions 
is  possible  than  in  the  spontaneous  increases  in  surface.  As  a 
matter  of  fact,  such  "critical"  points  have  long  been  recognized, 
especially  in  the  electric  dispersion  of  liquids  and  solids  (see 
above  and  later).  There  exists,  for  example,  a  so-called  "disin- 
tegration tension"  in  all  the  known  electric  methods  of  making 
colloid  solutions,2  at  which  the  dispersion  of  the  previously 
non-disperse  electrodes  suddenly  begins. 

Further,  the  critical  point  should  vary  with  the  value  of  the 
positive  surface  tension,  in  other  words,  with  the  value  of  the 
free  surface  energy  of  the  first  order  of  the  substance  to  be  dis- 
persed. As  a  matter  of  fact,  the  greater  the  positive  surface 
tension  of  the  substance  to  be  subdivided,  the  greater  is  the 

1  A  mathematical  formulation  of  the  conditions  necessary  for  dispersion  on  the 
basis  of  surface  energy  will  be  given  in  the  new  book  I  have  announced. 

2  See  Wo.  Ostwald,  Koll.  Zeitschr.,  7,  132  (1910). 


GENERAL   ENERGETICS    OF   THE    DISPERSOIDS  83 

amount  of  surface  energy  of  the  second  order  consumed,  in  other 
words,  the  greater  must  be  the  amount  of  electrical  energy,  for 
example,  that  must  be  introduced  into  the  system.  These 
deductions  are  supported  by  the  well-known  fact  that  progressive 
increases  in  surface  which  do  not  immediately  yield  disperse 
systems  are  observed  more  commonly  in  liquids  than  in  solids. 
As  a  rule,  large  quantities  of  energy  are  necessary  to  produce  an 
increase  of  surface  in  liquids  and  then  they  do  not  usually 
yield  disperse  systems  at  once.  As  already  mentioned,  only 
certain  solids  like  lead  show  progressive  increases  in  surface. 
We  may  explain  this  interesting  difference  between  solids  and 
liquids  by  the  well-known  fact  that  solid  phases  possess  a  greater 
positive  surface  tension  than  liquids  as  indicated  by  the  pro- 
gressive increase  in  surface  tension  of  cooling,  molten  substances. 
The  transitional  behavior  of  substances  like  lead  is  also  in  harmony 
with  this  view. 

When  we  apply  this  to  the  question  of  the  dispersive  effects 
of  equal  quantities  of  surface  energy  of  the  second  order  upon 
substances  having  different  positive  surface  tensions,  we  find  that 
the  greater  the  positive  surface  tension  of  the  substance  to  be  sub- 
divided, the  greater  the  degree  of  dispersion  of  the  system.  This 
brings  up  the  question:  under  what  circumstances  can  we  obtain 
the  highest  degree  of  dispersion  in  one  and  the  same  substance? 
The  answer  to  this  is  not  that  we  must  have  present  the  greatest 
possible  amount  of  free  surface  energy  of  the  second  order.  Were 
this  the  case  then  the  degree  of  dispersion  of  a  dispersoid  would 
have  to  be  proportional  to  its  solubility  and  this  is  by  no  means 
the  case.  To  produce  a  maximum  degree  of  dispersion  a  maximum 
of  free  surface  energy  of  the  first  order  must  also  exist  in  the  system, 
either  to  begin  with  as  in  solids,  or  as  the  result  of  an  especially 
great  increase  produced  through  an  increase  in  surface.  Hence, 
molecular-disperse  systems  will  be  formed  when  the  two  surface 
energies  acting  between  solvent  and  solute  attain  the  physical 
maximum.  To  the  important  consequences  of  this  characteriza- 
tion of  " molecules"  in  the  terms  of  surface  energies  for  our  con- 
ceptions of  the  structure  of  matter  we  shall  return  later  (see 
p.  96). 

Let  it  here   be  mentioned  that  F.  G.  Donnan1  following  a 

1  F.  G.  Donnan,  Z.  f.  physik.  Chem.,  37,  735  (1901);  46,  197  (1903). 


84  GENERAL  COLLOID -CHE  MIS  TRY 

suggestion  of  J.  H.  van't  Hoff,  has  constructed  a  capillary  theory 
of  colloid  solution  in  which  he  also  uses  the  concept  of  "nega- 
tive" surface  tension.  He  proceeds  from  the  fact  mentioned 
above  that  in  very  thin  layers  of  a  liquid  the  surface  tension  of  a 
particle  is  no  longer  independent  of  the  thickness  of  the  layer.  On 
the  basis  of  theoretical  considerations  which  originated  with 
Gauss,  he  concludes  that  in  layers  of  such  critical  thickness  the 
thicker  layers  tend  to  spread  and  become  thinner  while  the  thinner 
layers  tend  to  shrink  and  become  thicker.  The  resultant  consti- 
tutes an  equilibrium  yielding  the  stable  "  critical  particle."  It 
is  evident  that  this  interesting  theory1  differs  fundamentally 
from  that  outlined  above  in  that  the  sphere  of  action  of  the 
expansile  surface  tension  is  assumed  to  lie  only  within  the  "  layers 
of  critical  thickness"  or  the  "spheres  of  molecular  activities." 
According  to  our  view  one  may  observe  the  effects  of  expansile 
surface  tension  macroscopically,  just  as  one  may  observe  the 
effects  of  positive  surface  tension  in  coarsely  disperse  systems,  and 
all  independently  of  the  thickness  of  the  layers  of  particles  involved. 
Moreover,  Donnan's  view  compels  him  to  assume  a  qualitative 
difference  between  colloid  and  molecular-disperse  solutions  which 
is  unnecessary  in  our  conception.2  It  is  also  hard  to  conceive  of 
the  increases  in  surface  until  the  sphere  of  molecular  activities  is 
.  reached  in  Donnan's  theory.  "It  is  hard  to  conceive  just  what 
happens.  Apparently  the  solid  substance  spreads  into  O  (the 
solvent)  in  extremely  thin  layers  or  in  the  form  of  thin  branching 
threads.  It  should  be  noted  that  the  solid  colloid  is  not  in  an 
explosive  state,  for  dispersion  takes  place  only  in  the  thin  surface 
layers  so  that  the  process  of  ' solution'  of  the  colloid  need  not  be 
a  rapid  one,  etc."  (Donnan,  I.e.,  1901,  p.  738).  The  progressive, 
macroscopic,  microscopic  and  ultramicroscopic  increases  in  surface 
of  diphasic  systems  discussed  above  show  that  the  energetic 
theory  is  easily  capable  of  filling  this  gap. 

5.  Discontinuous  Diminutions  in  Surface. — When  one  discusses 
discontinuous  diminutions  in  surface  one  must  bear  in  mind  that  we 
deal  not  with  diminutions  in  the  surface  of  the  individual  particles 

1  It 'should  also  be  noted  that  F.  G.  Donnan  in  his  first  paper  (1901),  outlined  a 
more  kinetic  theory  of  colloid  solution  in  that  the  state  of  dispersion  was  regarded  as 
the  result  of  two  opposed  "molecular  streams"  occurring  in  the  surface.    These  proc- 
esses also  take  place  only  "within  the  molecular  spheres  of  action." 

2  See  p.  134  concerning  the  "saturation  point"  of  colloids  postulated  by  D( 


)onnan. 


GENERAL   ENERGETICS    OF    THE    DISPERSOIDS  85 

of  a  dispersoid  but  with  the  decrease  in  the  sum  of  the  surfaces  of  all 
the  particles  in  the  dispersion  means.  As  a  rule,  such  decreases  in 
total  surface  are  produced  by  approximation  or  coalescence  of  the 
individual  smaller  particles  into  larger  ones.  It  is  important  to 
note  that  such  decreases  need  not  take  place  only  through  con- 
densation (agglutination,  agglomeration,  coalescence,  etc.).  Slight 
decreases  in  surface  may  be  accomplished  when  for  any  reason 
elongated  or  flattened  particles  become  more  spherical.  Tend- 
encies toward  such  progressive  reductions  in  surface  are  en- 
countered when  dispersoids  are  cooled.  So  far  as  known,  the 
positive  surface  tension  between  two  (homogeneous)  phases 
always  increases  with  decrease  in  temperature.  Under  such 
conditions  irregularly  shaped  particles  would  therefore  tend  to 
become  more  spherical  with  fall  in  temperature.  Yet  the 
amounts  of  such  internal  diminutions  in  surface  would  at  all 
times  be  small. 

The  "condensation"  type  of  diminution  in  surface  is  important 
in  determining  the  properties  of  disperse,  more  particularly  of  col- 
loid systems.  It  shows  itself  in  coarsely  disperse  systems  in  the 
coalescence  of  emulsified  particles,  in  the  formation  of  threads  and 
flakes  from  microscopic  precipitates,  etc.  It  may  be  observed 
ultramicroscopically  in  colloid  systems  as  a  union  of  ultra- 
microns  to  form  crystalline  or  non-crystalline  particles.  In 
molecular-disperse  systems  the  process  of  "crystallization"  is 
encountered,  attained  at  times  only  after  passing  through  an 
intermediate  stage  (see  above).  Generally  speaking,  such  con- 
densations are  produced  by  the  same  means  which  accomplish 
their  dispersion,  only  different  intensities,  concentrations,  etc., 
have  to  be  used.  Thus,  while  electric  energy  has  a  dispersive 
effect,  removal  of  the  charge  leads  to  condensation,  especially  in 
colloids.  On  the  other  hand,  under  proper  circumstances  and 
with  certain  charges,  condensing  effects  may  be  accomplished 
electrically,  as  in  the  coalescence  of  electrified  droplets.1  Changes 
in  temperature  within  certain  limits  and  additions  of  foreign, 
especially  ionized,  substances  have  similar  effects.  Mechanical 
treatment,  like  sudden  one-sided  pressure,  may  also  bring  about 
condensation,  especially  in  coarsely  disperse  systems. 

When  we  study  a  simple  case  of  condensation,  as  the  coa- 

1  See  Lord  Rayleigh,  Proc.  Roy.  Soc.,  London,  28,  406  (1879);  34, 130  (1882). 


86  GENERAL  COLLOID-CHEMISTRY 

lescence  of  two  liquid  droplets,  we  find  it  hard  to  follow  the  transi- 
tion changes  from  the  original  state  to  the  final.  Coalescence 
usually  takes  place  very  rapidly  so  that  the  two  droplets  suddenly 
become  one,  though  it  may  still  be  possible  to  observe  the  con- 
tractile effects  of  the  positive  surface  tension  in  the  movements 
of  the  surface.  To  study  such  processes  of  condensation  in 
detail  it  is  best  to  use  drops  of  viscid  material1  which  consume  more 
time  in  the  process.  The  intermediate  phenomena,  which  we 
shall  find  of  special  theoretical  importance,  may  then  be  studied 
to  better  advantage. 

Intimate  contact  of  the  droplets  of  a  system  with  each  other 
seems  to  be  an  absolute  pre-requisite  for  coalescence  (as  well  as 
for  the  union  or  clumping  of  solid  particles).  The  droplets  must 
be  brought  so  close  together  that  their  surfaces  have  at  least  one 
"point"  in  common.  To  put  it  another  way,  condensation 
of  two  particles  can  occur  only  when  their  surfaces  are  continuous, 

O3  CO  O  O 

i  234 

FIG.  13. — Diagram  of  coalescence  of  two  fluid  particles. 
(According  to  L.  Michaelis.) 

even  though  such  surface  continuity  be  limited  to  a  single  point. 
It  should  be  noted  that  we  mean  a  "physical"  and  not  a  " mathe- 
matical "  point,  in  other  words,  a  structure  at  present  unmeasur- 
able  but  nevertheless  of  finite  dimensions.  Such  a  point,  greatly 
"magnified"  and  fixed  at  the  beginning  of  its  development  shows 
itself  as  a  cylindrical  or  doubly  coniform  neck,  as  illustrated 
schematically  in  Fig.  13  taken  from  L.  Michaelis.  This  con- 
necting piece  broadens  during  coalescence  until  complete  con- 
densation is  attained. 

Sometimes  (especially  in  viscid  and  in  solid  disperse  particles) 
another  type  of  contact  may  be  encountered  which  does  not 
correspond  with  the  description  just  given.  Here  the  particles 
also  approach  each  other  very  closely  but  do  not  come  in  direct 
contact.  In  other  words,  while  fixed  to  each  other  they  are 
nevertheless  still  separated  by  a  very  thin  layer  of  the  dispersion 
means.  The  adhesion  of  iron  filings  to  magnets  or  of  powders 
to  hot  objects,  etc.,  are  macroscopic  illustrations  of  such  contacts. 

1  See  J.  Loeb,  Koll.-Zeitschr.,  3,  113  (1908);  L.  Michaelis,  ibid.,  4,  55  (1909). 


GENERAL   ENERGETICS    OF    THE   DISPERSOIDS  87 

The  precipitation  of  coarse  suspensions  (of  kaolin,  quartz,  etc.) 
illustrates  the  same  phenomenon  in  a  disperse  system.  It  is 
probably  characteristic  of  such  "flakes"  that  the  individual 
particles  in  them  are  separated  from  each  other  by  a  distance  less 
than  the  diameter  of  the  surface  tension  films.  Figure  14  repre- 
sents the  matter  dia  grammatically.  Even  though  the  individual 
particles  in  this  type  of  contact  are  not  in  themselves  continuous, 
the  liquid  membranes  of  their  surfaces  are  (see  Fig.  14).  Be- 
cause of  this  difference  it  seems  well  to  distinguish  between  the 
two  and  to  designate  the  former  as  condensation  while  the  latter 
is  better  called  "aggregation."  Evidently  aggregation  may  often 
lead  to  condensation,  and  conversely  aggregation  may  be  assumed 
to  constitute  a  precursor  of  condensation. 


FIG.  14. — Diagram  illustrating  con-  FIG.  15.— Appearance   preceding  co- 

densation.  agulation  in  a  concentrated  gold  solu- 

tion.    (According  to  H.  Sidentopf.) 

As  is  well  known,  special  means  have  to  be  employed  to 
bring  about  such  intimate  contact  or  continuity  of  surfaces. 
One  of  the  chief  factors  which  tends  to  prevent  this  is  the  fact 
that  the  dispersion  means  (gas  or  liquid)  exists  at  the  phase 
surfaces  in  a  denser  state,  has  in  other  words  a  so-called  surface 
viscosity.  These  envelopes  act  like  the  vapor  envelopes  about 
the  drops  of  liquid  formed  when  water  is  poured  on  a  hot  surface; 
they  cause  a  "repulsion"  of  the  particles  when  they  meet  acci- 
dentally and  so  tend  to  prevent  their  coalescence.  These  phenom- 
ena are  closely  related  to  the  processes  of  "wetting"  touched  upon 
above.  Stress  was  laid  upon  the  importance  of  these  envelopes 
in  phenomena  of  condensation  early  in  the  history  of  colloid- 
chemistry.  Thus,  J.  M.  van  Bemmelen  wrote  in  1888:  "7  think 


88  GENERAL  COLLOID-CHEMISTRY 

it  possible  that  the  formation  of  the  flakes  which  are  precipitated  in  a 
liquid  is  dependent  upon  a  change  in  the  surface  tension  of  the  liquid 
membranes  surrounding  the  colloid  particles,  of  such  type  that  these 
membranes  between  the  particles  are  torn  at  some  point,  thus  per- 
mitting the  particles  to  form  aggregates."1 

The  condensation  of  disperse  particles  is  connected  with 
phenomena  of  movement  just  as  is  their  dispersion.  These 
"condensation  movements"  consist  of  a  mutual  approach  of  the 
particles  and  are  also  a  necessary  preliminary  for  their  contact 
and  coalescence.  In  fact  these  movements  precede  contact. 
The  first  demonstrable  changes  in  a  process  of  condensation  are 
therefore  kinetic  in  character.  This  fact  is  of  importance  for  the 
theory  of  condensation. 

The  appearance  of  such  condensation  movements  is  not  a 
mere  theoretical  assumption  but  a  necessary  conclusion  derived 
from  the  experimentally  observed  behavior  of  disperse  systems 
before  and  after  processes  of  condensation  have  occurred  in  them. 
Such  condensation  movements  have  actually  been  observed 
both  microscopically  and  ultramicroscopically  as  illustrated  in 
the  accompanying  Fig.  15  taken  from  Siedentopf.2 

6.  Theory  of  Condensation. — If  we  attempt  to  analyze  the 
processes  of  condensation  from  an  energetic  standpoint  as  was 
done  with  the  phenomena  of  dispersion,  we  discover  that  the 
former  are  more  complex.  In  dispersion  the  phenomena  of  sur- 
face energy  are  the  primary  ones,  while  processes  of  movement, 
the  formation  of  liquid  films,  etc.,  are  secondary.  But  in  the 
processes  of  condensation  these  different  secondary  phenomena 
must  take  place  in  reverse  order  before  the  surface  phenomena 
proper  come  into  play.  Such  considerations  harmonize  with  the 
fact  that  phenomena  of  condensation  and  the  means  of  initiating 
them  are  manifold  in  character  as  will  appear  later  when  we 
discuss  the  phenomena  of  coagulation.  The  theory  of  conden- 
sation therefore  divides  itself  into  two  parts,  first,  to  put  it 
briefly,  the  means  by  which  "intimate  con  tact "  of  the  particles  is 
brought  about,  and  second,  the  analysis  of  the  processes  taking 
place  after  contact  has  been  established.  Since  a  discussion  of  the 
different  means  by  which  the  intimate  contact  of  the  particles  is 

1  J.  M.  van  Bemmelen,  "Die  Absorption"  Gesammelte  Abhandl.,  22,  Dresden, 
1910.    The  citation  is  printed  in  italics  in  the  original  also. 

2  See  H.  Siedentopf,  Verh.  d.  Dtsch.  Physik.  Ges.,  1910,  25. 


GENERAL   ENERGETICS    OF    THE   DISPERSOIDS  89 

assured  belongs  to  the  field  of  special  dispersoid  and  colloid- 
chemistry,  this  must  be  postponed. 

Subject  to  general  discussion  here  are  the  changes  which  begin 
when  the  particles  of  a  dispersoid  begin  to  aggregate.  This 
process  is  characterized  by  the  formation  of  a  common  liquid  film 
about  the  particles.  Since  this  surface  film  grows  smaller  in  the 
process  of  aggregation,1  the  whole  seems  to  be  produced  through 
the  action  of  surface  energy  of  the  first  order.  It  is  also  clear  that 
with  any  increase  in  the  contractile  surface  energy  the  liquid  film 
tends  to  push  the  particles  closer  and  closer  together  until  they 
come  in  actual  contact.  When  we  deal  with  liquids,  coalescence 
of  the  particles  then  occurs  as 
described  above.  In  the  case 
of  solids  (provided  we  are  deal- 
ing with  actual  phenomena  of 
condensation  such  as  crystal 
formation)  there  is  a  coalescence 
of  at  least  the  solid  surface 
layers.  The  action  of  the  posi- 
tive  surface  energy  in  the  latter  FlG" 
case  may  be  imagined  as  shown 

in  Fig.  1 6.  There  results  in  all  instances  a  decrease  of  the  total  sur- 
face separating  the  disperse  phase  from  the  dispersion  means. 
The  process  of  condensation  is  therefore  to  be  regarded  as  the 
consequence  of  a  transformation  of  surface  energy  of  the  first  order. 
The  greater  the  condensation,  that  is,  the  smaller  the  resulting 
absolute  surface  separating  disperse  phase  from  dispersion  means, 
the  greater  the  amount  of  surface  energy  of  the  first  order  that 
has' been  transformed. 

Processes  of  condensation  do  not  always  yield  coarsely  disperse 
or  non-disperse  systems  but  may  stop  when  very  different  de- 
grees of  dispersion  have  been  attained  depending  upon  the  con- 
centration of  the  reaction  mixture,  as  shown  in  the  formation  of 
precipitates  in  chemical  reactions  (P.  P.  von  Weimarn).  This 
variety  in  degree  of  condensation  is  analogous  to  the  above-discussed 
variety  in  degree  of  dispersion  under  different  experimental  condi- 
tions, and  must  therefore  have  an  analogous  energetic  significance. 
The  degree  of  dispersion  in  a  condensing  system  depends  upon  the 

1  See  in  this  connection  the  citation  of  J.  M.  van  Bemmelen,  on  p.  88. 


QO  GENERAL  COLLOID-CHEMISTRY 

amount  of  expansive  surface  energy  present  in  it.  The  smaller  the 
surface  becomes  by  condensation,  the  greater  must  become  the 
tendency  of  the  expansive  surface  energy  to  counteract  the  diminu- 
tion of  surface.  The  system  becomes  stable  when  an  intermediate 
degree  of  dispersion  has  been  attained,  in  other  words,  when  the 
surface  energy  of  the  second  order  balances  the  surface  energy  of 
the  first  order  which  is  producing  the  condensation.  The  in- 
fluence of  the  introduction  of  other  forms  of  energy  upon  the 
degree  of  condensation  is  analogous  to  the  influence  of  these  as 
discussed  for  dispersion  on  p.  82. 

A  relation  between  condensation  in  colloids  and  surface  energies 
was  first  pointed  out  by  G.  Bredig1  in  explaining  a  special  form  of 
coagulation.  At  an  even  earlier  -date  P.  Curie2  pointed  out  the 
role  of  surface  energy  of  the  first  order  in  bringing  about  condensa- 
tion in  molecular-disperse  systems  in  processes  of  crystallization. 
It  is  remarkable  that  the  important  suggestions  of  this  investigator 
have  received  but  slight  (or  one-sided)  development  since  they 
were  first  expressed.  P.  P.  von  Weimarn  (whose  numerous 
papers  appear  in  the  Kolloid  Zeitschrift  and  in  the  Kolloid- 
Chemische  Beihefte)  has  also  developed  theories  of  condensation 
and  dispersion  which  he  believes  to  be  so  universally  applicable 
that  he  would  explain  through  them  all  known  processes  of  con- 
densation and  dispersion.  A  detailed  account  of  his  views  cannot 
be  given  here.  It  should,  however,  be  emphasized  that  no  theo- 
retical conception  of  such  processes  can  be  formulated  comparable 
in  universality  with  the  energetic  one  which  must  by  definition 
always  remain  the  broadest  form  in  which  natural  phenomena  may 
be  described.  P.  P.  von  Weimarn  in  his  theories  of  condensation 
and  dispersion  often  makes  use  of  moleculo-kinetic  conceptions, 
in  other  words,  he  employs  special  expedients  in  the  elaboration  of 
his  views.3 

1  G.  Bredig,  Anorg.  Fermente,  15,  Leipzig,  1901. 

2  P.  Curie,  Bull.  Soc.  Min.,  8,  145  (1885). 

*  Objections  may  be  raised  against  certain  details  of  the  argument  of  this  author. 
According  to  his  theory,  electrical  methods  of  pulverization  are  explainable  only  as 
condensation  processes,  which  is  obviously  wrong  in  view  of  the  dispersing  effects  of 
electrical  energy,  described  and  illustrated  on  p.  69.  Furthermore,  he  formulates 
the  basic  idea  of  his  theory  thus:  "When,  for  any  reason,  the  intensity  of  the  dis- 
solving forces  increases  on  the  surface  of  the  dispersed  particles,  but  does  not  exceed 
that  value  at  which  the  velocity  of  crystallization  or  of  solution  becomes  considerable, 
then  the  dispersed  particles  are  peptisized  (dispersed)  by  the  dispersion  means." 
[Kolloidchem.  Beih.,  1, 398  (1910)1.  I  can  see  in  this  only  a  "translation,"  and  not  an 


GENERAL   ENERGETICS    OF   THE   DISPERSOIDS  9 1 

§16.  Influence  of  the  Specific  Surface  upon  the  Relations  be- 
tween Surface  Energies  and  Other  Forms  of  Energy 

1 .  Specific  Surface  and  Volume  Energy ;  Capillary  Pressure. — 

The  relation  between  surface  energies  and  volume  energies  plays 
an  important  role  in  the  phenomena  observed  in  dispersoids.  If 
the  surface  energies  are  not  confined  to  a  plane  surface,  in  other 
words,  if  we  deal  with  structures  having  a  spatially  defined  surface 
or  one  which  is  curved,  then  the  two  surface  tensions  exert  pres- 
sure. To  put  it  more  correctly,  the  surface  energies  in  such 
bodies,  more  particularly  in  curved  surfaces,  readily  change  into 
volume  energies  when  opportunity  for  such  change  offers.  Thus, 
in  the  positive  surface  tension  of  a  markedly  curved  system  the 
centripetally  directed  capillary  pressure  may  bring  about  a  change 
in  the  pressure.  If  we  assume  the  particle  to  be  spherical,  the 
value  of  this  pressure  is  inversely  proportional  to  the  radius  and 
directly  proportional  to  the  surface  tension.  Analogous  phenom- 
ena are  encountered  when  the  curved  surfaces  have  a  negative 
surface  tension.  As  indicated  above1  these  relations  between 
surface  and  volume  energies  may  be  demonstrated  experimentally 
and  are  of  course  of  great  importance  in  dispersoids.  It  will  be 
shown  later  that  an  increase  in  density  due  to  positive  capillary 
pressure  may  be  demonstrated  experimentally. 

2.  Specific  Surface  and  Changes  of  State. — The  surface  energies 
which  dominate  the  behavior  of  disperse  systems  are  also  much 
influenced    by    temperature    (and    corresponding    herewith,    by 
pressure) .     Thus  the  vapor  pressure  of  small  droplets  or  particles 
is  found  to  be  greater,  at  a  given  temperature,  than  that  of  the  same 
substance   in   larger   masses.     Smaller   drops  therefore  tend  to 
evaporate  more  easily  than  larger  ones,  wherefore,  in  a  closed 
system  these  recondense2  upon  the  larger  ones.     A  lowering  of 
the  melting  point  of  solid  bodies  occurs  when  their  specific  surface 
is  increased,  just  as  does  a  decrease  in  the  evaporation  temperature 

analysis  of  the  process  of  dispersion,  for  the  assumption  of  "dissolving  forces"  and 
of  a  relation  of  these  to  other  processes  constitutes  the  problem  of  dispersion  but  does 
not  solve  it.  These  objections,  however,  are  not  valid  if  von  Weimarn's  theories  are 
limited  to  condensation  and  dispersion  phenomena  produced  by  chemical  means.  As 
will  become  later  evident,  the  theories  of  P.  P.  von  Weimarn  agree  throughout  with 
the  phenomena  observed  in  this  particular  field. 

1  Wilh.  Ostwald,  Grundr.  d.  allg.  Chem.,  4  Aufl.,  Leipzig,  1909,  p.  533. 

2  Wilh.  Ostwald,  Lehrb.  d.  allgem.  Chemie,  2  Aufl.  II,  2, 362;  Z.  f.  physik.  Chem. , 
22,  289  (1897). 


Q2  GENERAL  COLLOID-CHEMISTRY 

with  increasing  specific  surface.  Thus  P.  Pawlow1  found  dusts  of 
salol,  antipyrin,  etc.,  to  melt  at  a  temperature  some  7°  lower  than 
larger  particles.  He  calculates  that  in  the  case  of  salol,  a  depres- 
sion of  the  melting  point  of  2.8°  about  corresponds  to  a  hundred- 
fold increase  in  specific  surface.  A  far-reaching  influence  of  the 
specific  surface  or  curvature  is  indicated  also  in  the  phenomena  of 
solidification  or  freezing  of  homogeneous  systems.  According  to 
Muller-Thurgau,2  filter  paper,  moistened  with  distilled  water, 
freezes  at  —  0.1°,  while  a  clay  sphere,  moistened  with  water, 
freezes,  according  to  Bachmetjew,3  at  —0.7°.  These  figures  are 
not  simply  so  called  under-cooled  values  for  water,  but  indicate 
freezing  temperatures  after  such  under-cooling  is  eliminated. 

In  these  processes  of  evaporation,  of  melting  and  freezing,  a 
number  of  energies  change.  Positive  and  negative  changes  in 
volume  and  density  take  place,  solid  bodies  acquire,  on  melting, 
free  surface  energies  of  the  first  order,  the  optical  properties  change, 
etc.  For  these  reasons  it  is,  as  yet,  not  possible  to  show  the 
relations  which-  exist  between  single  energy  changes  and  the 
simultaneously  appearing  changes  in  the  surface  energies,  the  effect 
of  which  increases  with  increasing  specific  surface. 

3.  Specific  Surface  and  Electrical  Energy. — The  relations 
between  electrical  energy  and  surface  energies  must  also  change 
when  macro-heterogeneous  are  compared  with  disperse  systems. 
Th.  Des  Coudres4  showed  that  in  harmony  with  our  theory,  a 
difference  of  potential  between  curved  and  flat  surfaces  of  mercury 
may  not  only  be  proved  experimentally  but  its  value  be  ap- 
proximately calculated.  Of  the  influence  of  an  electrical  potential 
opposing  the  positive  surface  tension,  O.  Lodge5  states  that  in  a 
drop  this  influence  increases  inversely  as  the  fourth  power  of  the  diam- 
eter of  the  drop.  In  this  connection  should  also  be  mentioned  the 
important  study  of  H.  von  Stein wehr6  who  found  that  finely  ground 
calomel,  as  used  in  the  preparation  of  normal  electrodes,  shows 
a  greater  difference  of  potential  toward  its  saturated  solution  than 
does  the  same  substance  when  less  highly  dispersed.  Further 

1  P.  Pawlow,  Z.  f.  physik.  Chem.,65,  i,  545  (1909);  74,  562  (1910);  Koll.-Zeitschr. 
6,  37  (1910);  7,  265  (1910);  P.  P.  von  Weimarn,  ibid.,  6,  32  (1910);  7,  205  (1910). 

2  Mtiller-Thurgau,  Landwirtschaftl.  Jahrb.,  9,  176  (1880). 

3  Bachmetjew,  Z.  f.  wissensch.  Zoologie,  66,  584  (1899). 

4  Th.  Des  Coudres,  Wiedem.  Ann.  d.  Physik.  46,  292  (1892). 

*  Wm.  C.  Me.  C.  Lewis,  Koll.-Zeitschr.,  5,  91  (1909);  also  E.  Hatschek  ibid.,  7, 
158  (1910). 

8  H.  von  Steinwehr,  Z.  f.  Instrumentenkunde,  25,  205  (1906). 


GENERAL  ENERGETICS   OF   THE   DISPERSOIDS  93 

relations  between  the  value  of  the  specific  surface  of  electrodes 
and  electrochemical  phenomena  may  be  found  in  the  paper  of 
G.  Bredig  and  J.  Teletow.1 

We  would  expect,  on  general  principles,  that  the  relations 
between  surface  energies  and  electrical  energy  would  play  an 
especially  important  part  in  the  case  of  dispersoids.  The  majority 
of  electrical  phenomena  take  place  on  the  surface  since  electrical 
energy,  in  contrast  to  heat,  for  example,  tends  to  reside  on  the 
surface  of  a  homogeneous  body.  The  electrical  capacity  of  a 
hollow  metal  condenser  is  therefore  about  as  great  as  that  of  a 
correspondingly  large  solid  body.  Electrical  energy  will  therefore 
often  enter  easily  into  reciprocal  action  with  the  surface  energies. 
The  great  importance  of  these  electrical  phenomena  in  colloid 
systems  will  become  apparent  in  the  special  parts  of  this  book. 

4.  Specific  Surface  and  Chemical  Energy.— Since  colloids 
belong  to  the  heterogeneous  systems,  the  general  law  of  chemical 
kinetics  governing  such  systems,  may  be  applied  to  them.  This 
states  that  the  amount  of  chemical  change  in  the  unit  of  time  is 
proportional  to  the  absolute  surface  (Wenzel).2  This  leads  one  to 
suspect,  because  of  the  extraordinarily  large  absolute  surface  in 
colloids,  that  many  reactions  will  occur  more  rapidly  in  them  than 
in  coarse  heterogeneous  systems.  Such  is,  in  fact,  true.  M.  Raffo 
and  A.  Pieroni3  found  that  colloid  sulphur  behaved  toward  silver 
salts  like  an  energetic  reducing  agent;  while  non-colloid  sulphur, 
even  though  finely  divided  and  obtained  by  precipitation  of  a 
polysulphide,  would  not  form  silver  sulphide  in  the  cold.  Even 
after  prolonged  boiling  this  occurred  only  partially.  The  reactions 
of  precipitated  metallic  silver  vary  according  to  .the  size  of  its 
particles.  The  coarsely  dispersed  "gray"  silver,  obtained  by 
reduction  with  oxalates,  is  less  sensitive  to  mercuric  chloride 
than  is  the  highly  dispersed  "black"  silver,  precipitated  by 
sulphites,  etc.  (R.  Liesegang,  Liippo-Cramer).4  Analogous  rela- 
tions exist  in  the  decomposition  of  hydrogen  peroxide  by  platinum. 
While  smooth  platinum  foil  decomposes  this  compound  slowly,  a 
"platinized"  foil  (one  covered  with  finely  divided  metallic 
platinum)  does  it  more  rapidly.  When  colloid  platinum  is 

1  G.  Bredig  and  J.  Teletow,  Z.  f.  Elektroch.,  12,  589  (1906). 

2  See  Wilh.  Ostwald,  Grund.  d.  allg.  Chemie,  4  Aufl.,  328,  Leipzig,  1909. 

3  M.  Raffo  and  A.  Pieroni,  Koll.-Zeitschr.,  7,  158  (1910). 

4  Liippo-Cramer,  Koll.-Zeitschr.,  3,  35  (1908). 


94  GENERAL  COLLOID-CHEMISTRY 

used,  the  effect  is  still  observable,  if  there  is  but  i  gram-atom 
of  platinum  in  70  million  liters  of  the  reaction  mixture  (or  i 
gram-atom  of  colloid  palladium  in  26  million  liters;  or  i  gram- 
atom  of  colloid  gold  in  one  million  liters).1 

Still  greater  surface  effects  are  naturally  to  be  expected  when, 
as  in  the  last  example,  we  deal  with  phases  having  different  specific 
surfaces,  that  is,  having  different  surface  concentrations  in  space. 
From  the  existence  of  capillary  pressure  and  from  the  changes  in 
density  which  result  from  this  pressure  we  would  expect  an  in- 
fluence upon  the  velocity  of  chemical  reactions,  for  the  speed  of  a 
chemical  reaction  is  primarily  dependent  on  the  density  of  con- 
centration of  the  reacting  components.  Therefore,  we  would 
expect  that  the  phenomenon '  of  catalysis  would  be  especially 
marked  in  colloid  systems.  The  distinguishing  characteristic 
of  a  catalyzer  resides  in  the  enormous  change  which  it  is  capable 
of  bringing  about  in  the  velocity  of  a  chemical  reaction.  Thanks 
to  the  brilliant  investigations  of  G.  Bredig,2  his  students  and  others, 
it  has  been  shown  that  many  catalytic  effects  may  be  brought 
about  by  highly  dispersed  surfaces  of  all  kinds,  and  that  the 
especially  important  catalytic  reactions  of  the  organic  ferments 
may  be  closely  imitated  by  various  inorganic  materials  in  the 
colloid  state,  such  as  the  colloid  metals.  We  need  in  illustration 
but  recall  the  catalytic  effects  on  gases  of  a  trace  of  platinum 
sponge,  or  platinum  black  as  compared  with  the  effects  of  a  piece 
of  smooth  platinum  foil.  The  great  part  played  here  by  the 
specific  surface,  that  is,  the  volume  concentration  of  the  surface,  is 
also  self- apparent. 

1  G.  Bredig,  Bioch.  Zeitschr.,  6,  315  (1907);  G.  Bredig  and  J.  Teletow,  Z.  f.  Elek- 
troch.,  12,  581  (1906);  J.  Teletow  (abstract),  Chem.  Centr.,  i,  793  (1908). 

2  G.  Bredig,  Anorganische  Fermente.,  Leipzig,  1901 ;  further,  the  recent  review  of 
the  author  in  Bioch.  Zeitschr.,  6,  283  (1907) ;  here  may  also  be  found  many  references  to 
the  literature.  The  following  according  to  Bredig,  are  the  best  connected  presentations 
of  the  field.     Bodlaender,  Uber  langsame  Verbrennung,  Stuttgart,  1899.  W.  Ostwald, 
Grundr.  d.  allgem.  Chem.,  1909;  Leitlinien  der  Chemie,  1906;  Uber  Katalyse,  Leip- 
zig, 1902;  Natur-philosophie,  1902.     Sv.  Arrhenius,  Immunochemie,  Leipzig,  1907; 
Theorien  der  Chemie,  Leipzig,  1906.     W.  Nernst,  Theoret.  Chemie,  1909.     W.  Herz, 
Lehre  von  der  Reacktionsbeschleunigung.,  Stuttgart,  1906.    R.  Hoeber,  Physikalische 
Chemie  der  Zelle  u.  d.  Gewebe,  Leipzig,  1906.     E.  Cohen,  Physical  Chemistry  for 
Physicians  and  Biologists,  Trans,  by  M.  H.  Fischer,  New  York,  1903;  H.  J.  Ham- 
burger, Osmotischer  Druck  u.  lonenlehre  i.   d.   mediz.   Wiss.,  Wiesbaden,  1904. 
G.  Bredig,  Elemente  der  chemischen  Kinetik,  in  Spiro  u.  Ashers  Ergeb.  d.  Physiol., 
1902.     Schade,  Bedeutung  der  Katalyse  in  der  Medizin,  Kiel,  1907.     M.  Bodenstein 
Chem.-Zeitg.  26,  1075,  1902.     J.  W.  Mellor,  Chemical  Statics  and  Dynamics,  Lon- 
don 1904;  H.  Freundlich,  Kapillarchemie,  Leipzig,  1909.     Comprehensive  presenta- 
tions by  Bredig  appear  in  Oppenheimer's  Handb.  d.  Bioch.  as  well  as  in  Bredig's 
Handb.  d.  angewandt.  physik.  Chemie. 


GENERAL   ENERGETICS    OF   THE   DISPERSOIDS  9$ 

Closely  connected  with  density  changes  of  great  surfaces  are 
the  so-called  adsorption  phenomena,  which  we  shall  consider  in 
detail  later.  With  these  are  also  connected  changes  of  a  chemical 
nature  and  reaction  accelerations.  But  since  they  cannot  be 
discussed  to  advantage  without  a  previous  discussion  of  adsorp- 
tion itself,  we  must  postpone  the  whole  matter.  Even  now, 
however,  we  may  point  out  that  theoretically  the  amount  of  a 
reaction  product  ultimately  obtained,  in  other  words,  the  equilib- 
rium point  in  a  chemical  reaction,  -may  be  shifted  under  the 
influence  of  great  spatial  concentrations  of  the  surface  energies, 
as  obtaining  in  dispersoids,  for  example.1  If  a  chemical  reaction 
occurs  in  the  zone  of  contact  between  two  phases,  in  which,  for 
example,  a  positive  surface  tension  is  present,  either  of  two  things 
may  happen.  The  surface  tension  may  be  either  raised  or  lowered 
by  the  chemical  change  occurring  in  the  two  phases.  In  the  first 
instance,  the  "  chemical  resistance,"  that  is,  the  speed  of  the  oppos- 
ing reaction,  would  be  decreased  through  the  consumption  of 
energy  necessary  for  the  increase  in  the  surface  tension;  in  the 
second,  wherein  the  surface  tension  diminishes,  an  acceleration 
of  the  reaction  would  occur,  for  the  free  surface  energy  produced 
would  now  tend  to  change  into  chemical  energy.  Besides  the 
increase  in  rate,  there  would  also  be  an  increase  in  the  product  of 
the  reaction,  since  the  amount  of  chemical  energy  available  for 
its  formation  is  increased  by  the  amount  resulting  from  the 
transformation  of  surface  energy  into  chemical  energy.  A  great 
specific  surface  will  therefore  be  able  to  shift  the  equilibrium 
point  of  a  chemical  reaction  just  as  does  a  rise  in  temperature. 
Wilh.  Ostwald1  has  given  a  practical  illustration  of  this.2  If 
the  solution  of  a  salt  of  a  fatty  acid  is  brought  in  contact  with  a 
large  surface,  the  fatty  acid  set  free  by  hydrolysis  tends  to  collect 
in  the  surface,  that  is,  it  concentrates  itself  there  more  than  does 
the  base.  The  hydrolytic  equilibrium  of  the  remaining  solu- 
tion is  thereby  disturbed,  and  to  reestablish  it,  more  of  the 

1  J.  J.  Thomson,  Applications  of  Dynamics  to  Physics  and  Chemistry,  203,  234, 
London  (1888);  see  also  the  extensive  discussion  of  this  question  but  not  one  free  from 
objection,  by  T.  B.  Robertson,  Koll.-Zeitschr.,  3,  49  (1908),  and  succeeding  pages, 
especially,  Part  III.     That  the  osmotic  equilibrium  between  two  molecular  disper- 
soids, and  that  the  distribution  of  a  molecularly  dispersed  substance  between  two 
phases  depends  on  the  specific  surface  of  the  phases  has  been  proved  theoretically  by 
F.  Kaufler,  Zeitschr.  f.  physik.  Chem.,  43,  686  (1908). 

2  Wilh.  Ostwald,  Z.  f.  physik.  Chem.,  62,  512  (1908). 


96  GENERAL  COLLOID-CHEMISTRY 

salt  must  hydrolyze.  Other  phenomena  of  this  class,  especially 
as  observed  in  colloid  systems,  will  be  discussed  later. 

Finally,  it  should  be  noted  that  several  exceptions  have  been 
noted  to  the  general  rule  that  substances  with  large  specific 
surfaces  react  more  rapidly  than  coarsely  dispersed  ones.  Mc- 
Intosh1  states  that  colloid  silver  dissolves  very  slowly  in  acids. 
Its  solution  can  be  greatly  accelerated  by  the  addition  of 
small  amounts  of  permanganate.  One  is  inclined  to  suspect 
the  presence  of  silver  oxide  coatings  over  the  metallic  particles 
in  this  case,  which  interfere  with  the  action  of  the  acid,  rather 
than  to  suppose  this  to  be  an  actual  exception  to  WenzePs  law. 
Occasionally  in  the  literature  of  colloid-chemistry,  we  encounter 
the  statement  that  colloid  solutions  react  " sluggishly."  In  the 
light  of  our  discussion,  this  statement  is  not  correct  when  compari- 
son is  made  between  colloidally  and  coarsely  dispersed  systems. 
But  when  comparison  is  made  with  the  reactivity  of  molecular 
and  ionic  dispersoids,  it  is.  It  has  been  proved  with  any  two 
substances  composing  a  dispersoid  that  the  reactivity  decreases 
progressively  with  decreasing  degree  of  dispersion.  In  molecular 
and  ionic  dispersoids  in  which  it  might  be  said  that  the  dis- 
perse particles  consist  "  almost  entirely  of  surface,"  one  would 
therefore  expect  an  enormous  development  of  surface  energies. 
In  this  connection,  one  is,  as  a  matter  of  fact,  reminded  of  the  old 
chemical  saying,  "Corpora  non  agunt  nisi  soluta."  But  the  part 
played  by  the  chemical  energy  resulting  from  the  conversion  of 
surface  energies  during  chemical  reactions  in  disperse  systems, 
must  also  decrease  with  increasing  degree  of  dispersion.  When 
we  come  to  deal  with  maximum  degrees  of  dispersion,  in  other 
words,  with.  " indivisible"  particles  such  as  molecules,  atoms  or 
even  electrons,  one  might  develop  a  conception  according  to  which 
chemical  reactions,  that  is,  the  union  and  separation  of  molecules 
or  atoms,  etc.,  represent  merely  the  results  of  decreases  in  the 
surfaces  of  the  particles  involved.  The  dynamics  of  molecules 
and  atoms  and  especially  the  effects  of  chemical  energy  can  in 
this  sense  come  to  be  viewed  as  mere  manifestations  of  the  surface 
energies  of  maximally  dispersed  particles.  The  discontinuity  of 
matter  in  which  we  have  always  believed  and  which  has  been 
proved  in  various  ways  then  becomes  synonymous  with  the  ex- 

1  Mclntosh,  Amer.  Journ.  Physic.  Chem.,  6,  17  (1902). 


GENERAL   ENERGETICS    OF   THE   DISPERSOIDS  Q7 

istence  of  an  immensely  great  absolute,  as  well  as  specific  surface; 
and  all  changes  in  this  discontinuity  become  connected  with 
changes  in  the  amount  of  the  surface  of,  or  of  the  degree  of  dis- 
continuity in  the  substance,  in  other  words,  with  changes  in  the 
capacity  factors  as  well  as  the  spatial  concentrations  of  the  surface 
energies.1 

5.  Specific  Surface  and  Radiant  Energy. — The  connection 
between  specific  surface  and  another  type  of  energy,  namely, 
radiant  energy,  is  closely  related  to  the  chemical  phenomena 
discussed  in  the  previous  division.  Stas2  found  that  the  photo- 
chemical sensitiveness  of  silver  chloride  precipitates  increased  with 
their  degree  of  dispersion.  Corresponding  to  the  series  given  on 
p.  75,  the  sensitiveness  to  light  increased  from  the  granular, 
through  the  powdered,  up  to  the  flocculent  or  cheesy.  Interest- 
ingly enough,  Stas  emphasized  that  it  is  the  latter  type  and  not  the 
"gelatinous"  state  of  silver  chloride  which  is  most  sensitive  to 
light.  Were  we  to  assume,  as  does  P.  P.  von  Weimarn,  that  the 
gelatinous  is  only  a  continuation  of  the  other  varieties  of  pre- 
cipitates, in  the  sense  that  the  precipitate  in  the  gelatinous  form 
represents  merely  a  still  finer  division  of  the  particles,  but  is 
otherwise  of  the  same  general  character,  is  crystalline,  for  ex- 
ample, then  the  behavior  observed  by  Stas  would  constitute  a 
contradiction  of  Wenzel's  law.  But  not  only  the  improbability 
of  such  an  exception  but  many  other  reasons  indicate  that  in 
" gelatinous"  silver  chloride  we  are  dealing  with  a  system  fun- 
damentally different  from  that  characterizing  the  other  solid 
precipitates.  It  is  an  emulsoid  in  contrast  to  the  others  which 
are  suspensoids. 

These  relations  between  photochemical  sensitiveness  and  size 
of  granules  have  often  been  observed  since  Stas's  work  and  have 

1  The  history  of  science  teaches  that  we  have  always  held  to  the  theory  of  the  dis- 
continuity of  matter,  but  that  different  kinds  of  energy  were  in  turn  made  responsible 
for  or  associated  with  the  elementary  changes  in  the  discontinuity.     Distance  energy 
(attracting  and  repelling  forces)  kinetic  energy,  and  more  recently,  electrical  energy 
have  in  turn  been  associated  with  the  discontinuity.    It  is  of  interest  to  point  out 
that  this  electrical  theory  of  the  structure  of  matter  is  closely  allied  with  the  concept 
that  the  surface  energies  are  the  forces  responsible  for  the  elementary  changes 
in  discontinuity,  for,  as  pointed  out  above,  electrical  phenomena  occur  chiefly  on 
surfaces.     It  seems,  therefore,  but  a  further  step  in  the  same  direction,  if   we 
add  surface  tension  and  surface  energies  to  the  "forces"  already  considered,  since 
both  of  them  are  as  widely  distributed  and  important  as  the  discontinuity  of  matter 
itself. 

2  Stas,  see  K.  Drucker,  Koll.-Zeitschr.,  4,  216  (1909). 

7 


98  GENERAL  COLLOID-CHEMISTRY 

attained  great  importance  in  the  practice  of  photography  and  in 
the  preparation  of  photographic  films.1 

A  more  interesting,  and  perhaps  more  important  discovery  is 
the  unusual  one  of  E.  Wedekind  and  H.  Baumhauer2  (together 
with  Gockel)  that  the  emanations  of  radio-active  substances 
may  be  much  increased  if  they  are  highly  dispersed,  as  by  being 
converted  into  colloid  form.  These  authors  succeeded  in  preparing 
radio-active  thorium  in  colloid  form.  A  comparison  of  the  radio- 
activity of  this  thoriumsol  with  that  of  the  metallic  (coarsely 
dispersed)  element,  measured  by  the  volt  decrease  per  hour, 
showed  the  surprising  fact  that  the  radio-activity  of  a  sol  containing 
only  0.0235  gram  was  equal  to  that  of  a  coarsely  dispersed  sus- 
pension containing  o.iu  gram.  In  other  words,  the  radio-activity 
of  the  sol  was  4.8  times  as  great  as  that  of  the  coarsely  dispersed 
element. 

The  extraordinary  significance  of  this  discovery3  lies  in  the  fact 
it  has  not  as  yet  proved  possible  to  influence  markedly  the  emana- 
tion from  a  radio-active  substance  by  any  other  means4  as  by 
raising  the  temperature,5  evacuation,  electrolysis,  etc.  A  more 
striking  demonstration  of  the  great  effect  of  the  surface  energies 
which  come  into  play  with  increase  in  dispersion  could  scarcely  be 
found  than  this  singular  effect  of  degree  of  dispersion  upon  the 
radio-active  dissolution  of  the  elements.  Furthermore,  this  fact 
seems  to  indicate  that  the  surface  energies  will  come  to  play 
not  only  an  important,  but,  in  comparison  with  the  other  kinds  of 
energy,  perhaps  a  dominant  part  in  a  general  theory  of  matter.6 

1  See  Luppo- Cramer,  Kolloidchemie  u.  Photographic  (Dresden  1908)  as  well  as 
the  numerous  papers  of  this  author  in  the  Kolloid  Zeitschrift. 

2  E.  Wedekind  and  H.  Baumhauer,  Koll.-Zeitschr.,  5,  192  (1909). 

3  It  was  not  recognized  by  the  authors  themselves. 

4  See  the  Textbooks  on  Radio-activity. 

6  Recently  an  insignificant  influence  of  temperature  has  been  observed  (Engler, 
etc.). 

6  It  would  be  a  feat  in  colloid  chemistry  to  carry  out  analogous  experiments  with 
colloid  radium  salts.  Since  colloid,  especially  suspensoid  systems  exhibit  their 
characteristic  properties  with  even  minimal  amounts  of  disperse  phase,  only  small 
amounts  of  radium  salts  would  be  necessary.  One  might  first  test  out  their  prepa- 
ration by  using  the  physico-chemically  similar  barium  salts,  and  after  having  dis- 
covered a  suitable  "micro-chemical"  method  apply  it  to  radium.  Gelatinous 
radium  salts  could  perhaps  be  prepared  by  methods  analogous  to  those  used  by  C. 
Neuberg  and  his  students  (Koll.-Zeitschr.,  2,  321,  354)  on  barium  salts  in  alcoholic 
solvents. 


CHAPTER  IV 

DISTRIBUTION  OF  THE  COLLOID  STATE  AND  THE 
CONCEPT  OF  COLLOID  CHEMISTRY 

§17.  The  Fundamental  Independence  of  the  Colloid  State  of  the 
Chemical  Nature  of  the  Phases 

i.  Statistical  and  Experimental  Development  of  the  Idea  of 
the  Universality  of  the  Colloid  State. — In  the  forthcoming  historical 
portion  of  this  work  it  will  be  shown  that  the  number  of  known 
colloid  systems  has  steadily  increased  as  colloid  chemistry  has 
developed.  In  Graham's  time  (1861)  and  even  later,  colloidality 
was  generally  held  to  be  characteristic  of  certain  substances,  but 
with  the  discovery  of  general  methods  of  preparing  colloid  systems, 
it  soon  became  clear  that  this  was  too  narrow  a  viewpoint.  At  the 
present  time,  we  may  say  that  practically  all  solid  substances  have 
been,  or  can  be  prepared  in  colloid  form  by  some  method  or  other. 
P.  P.  von  Weimarn,  for  example,  has  by  a  single  method  "  con- 
verted" over  two  hundred  different  substances  (salts,  elements, 
etc.)  into  colloids.  Of  course,  different  substances  are  changed 
into  the  colloid  condition  with  different  degrees  of  ease,  but  no 
decisive  effect  of  the  chemical  nature  of  the  substance  whose 
dispersion  is  attempted  has  as  yet  been  discoverable. 

Nor  is  the  chemical  nature  of  a  dispersion  means  of  basic  signifi- 
cance in  determining  its  ability  to  maintain  a  second  substance  in 
the  colloid  condition.  Even  Graham  knew  that  different  disper- 
sion media  could  mutually  displace  each  other  without  destroying 
the  colloid  state.  He  was  able  to  replace  the  water  of  a  silicic 
acid  gel  with  alcohol,  with  sulphuric  acid,  etc.  And  while  the  first 
known  metallic  colloids  were  hydrosols,  many  metallic  organosols 
(metallic  colloids  in  various  organic  dispersion  means)  have 
recently  been  prepared.  Among  these  are  the  sols  of  the  alkali 
metals  which  cannot  even  exist  in  water  (The  Svedberg). 

Neither  is  the  suspensoid  or  emulsoid.  character  of  a  colloid 

99 


100  GENERAL  COLLOID-CHEMISTRY 

determined  by  the  chemical  nature  of  the  disperse  phase.  There 
exist  inorganic  as  well  as  organic  suspensoids.  Generally  speak- 
ing, the  emulsoid  states  are  more  common  than  the  suspensoid, 
in  the  case  of  albumins,  for  example;  but  suspensoids  are  also 
found  among  these,  as  shown  by  their  ready  precipitability  through 
traces  of  electrolytes,  by  their  low  internal  friction,  etc.  (see 
pp.  12,  13).  As  P.  P.  von  Weimarn  has  shown  in  his  fun- 
damental researches,  the  same  substance  may  be  obtained  either 
in  the  suspensoid  or  emulsoid  state  (as  a  jelly)  depending  upon 
the  conditions  of  its  preparation.  One  and  the  same  substance 
may  also  exhibit  either  a  suspensoid  or  an  emulsoid  charac- 
ter depending  upon  the  nature  of  the  dispersion  means,  as 
Freundlich  and  Neumann  have  found  in  the  case  of  dyes  (see 

P.  56). 

Finally,  one  and  the  same  substance  may  appear  under  different 
circumstances  either  as  a  crystalloid  or  a  colloid.  We  need  but 
recall  the  crystallization  of  albumin  or,  on  the  other  hand,  the 
production  in  colloid  form  of  materials  usually  known  only  as 
crystalloids,  such  as  common  salt.1  P.  P.  von  Weimarn  (I.e.) 
recently,  showed  that  mere  change  in  the  concentration  of  the  com- 
ponents of  a  reaction  mixture  sufficed  to  precipitate  them  either 
in  colloid  or  crystalloid  form.  These  facts  show  clearly  the  fun- 
damental independence  of  the  colloid  (and  crystalloid)  state,  of 
the  special  chemical  properties  of  the  substances  involved. 

Obviously,  the  growing  acquaintance  of  investigators  with 
new  colloid  materials  could  not  help  but  lead  them  gradually  to 
recognize  that  colloid  properties  were  not  confined  to  specific 
chemical  substances.  The  attempts  of  P.  Rohland2  in  1907  to 
tabulate  colloid  materials  showed  clearly  how  impossible  was  such 
a  chemical  viewpoint.  The  result  was  entirely  unsatisfactory, 
for  the  table  included  not  only  a  heterogeneous  lot  of  chemical 
substances,  but  was  incomplete.  Conversely,  however,  it  dem- 
onstrated the  impossibility  of  coordinating  satisfactorily  chemical 
composition  with  colloid  properties  and  brought  home  the  fact 
that  all  materials  may  occur  in  the  colloid  state.  But  while  this 
view  was  already  beginning  to  be  recognized  in  1905  as  a  nec- 
essary conclusion  to  be  drawn  from  the  rapidly  increasing  list  of 

1  C.  Paal,  Ber.  d.  D.  chem.  Ges.,  39,  1436,  2859,  2863  (1906). 

2  P.  Rohland,  Koll.-Zeitschr.,  i,  201,  289  (1907);  2,  S3  (1907)- 


DISTRIBUTION    OF    THE    COLLOID    STATE  IOI 

colloid  materials1  it  should  be  emphasized  that  P.  P.  von  Wei- 
marn  (1906)  was  the  first  to  express  clearly  and  emphatically  on 
the  basis  of  these  findings  that  the  colloid,  like  the  crystalloid,  is  a 
universally  possible  state  of  matter. 

Although  experiment  shows  the  colloid  state  to  be  independent 
of  the  chemical  composition  of  the  phases,  this  does  not  of  course 
mean  that  the  properties  of  the  dispersoids  may  not  change  with 
varying  chemical  composition  of  the  phases.  Examples  have 
already  been  given  which  show  that  one  and  the  same  chemical 
substance  may  assume  different  types  of  dispersion  with  dif- 
ferent kinds  of  dispersion  media.  The  usual  view  of  this  be- 
havior which  holds  the  " chemical  nature"  of  the  phases  respon- 
sible for  the  observed  changes,  may  easily  lead  to  error,  for  it 
is  not  the  chemical  properties,  in  other  words,  the  analytical 
composition  and  the  reactivity  which  determines  that  a  sub- 
stance dissolves  as  a  colloid  or  molecular  dispersoid,  but  rather 
the  different  physical  properties  such  as  different  " solubility" 
values,  etc.,  in  other  words,  the  free  surface  energies  which  bring 
about  the  variations  in  degree  of  dispersion.  Of  course,  these 
physical  properties,  like  other  properties  are  in  good  part  de- 
pendent upon  the  chemical  composition  of  the  phases,  and  so 
change  with  chemical  changes  in  these.  Obviously  the  sta- 
bility, reactivity,  etc.,  of  a  colloid  must  therefore  vary  with  changes 
in  the  chemical  composition  of  the  phases  concerned.  But  the 
chemical  relations  between  disperse  phase  and  dispersion  means 
characterize  the  dispersoid  just  as  little  as  the  absorption  or 
liberation  of  heat  which  always  accompanies  chemical  processes 
completely  characterize  these,  even  though,  as  is  well  known, 
temperature  influences  them  greatly. 

2.  Universality  of  the  Colloid  State  as  a  Necessary  Con- 
sequence of  Characterizing  Colloid  Solutions  as  Disperse 
Systems.— If  it  is  granted  that  colloid  solutions  are  merely  repre- 
sentatives of  disperse  systems  and  that  their  properties  are  deter- 
mined through  a  degree  of  dispersion  which  has  both  an  upper  and  a 
lower  limiting  value,  it  becomes  self-evident  that  almost  any  desired 
material  may  be  prepared  in  the  colloid  condition.  For  as  certainly 
as  all  substances  have  not  an  unlimited  solubility  in  every  solvent, 

1  In  this  connection  see  Wo.  Ostwald,  Koll.-Zeitschr.,  6, 184  (1910) ;  R.  Zsigmondy, 
zur  Erkenntnis  der  Kolloide,  pp.  170,  171,  175,  Jena,  1905. 


102  GENERAL  COLLOID-CHEMISTRY 

equally  certainly  can  these  substances  be  gotten  into  a  disperse  form 
provided  a  proper  dispersion  means  is  chosen.  For  every  substance 
a  second  may  be  found  in  which  the  first  is  "insoluble"  or  only 
slightly  "soluble."  All  special  problems  in  "colloid  synthesis" 
consist  in  finding  the  experimental  means  of  obtaining  the  average 
values  in  degrees  of  dispersion  that  are  characteristic  of  colloids, 
or  of  fixing  such  in  passing  through  a  series  of  progressively 
changing  degrees  of  dispersion.  The  question  of  the  "possibility 
of  all  substances  existing  in  a  colloid  state"  stands  and  falls  with 
this  recognition  of  colloid  solutions  as  mere  examples  of  dispersed 
heterogeneous  systems. 

It  is  evident  that  this  classification  of  colloid  and  dispersed 
systems  accepts  from  the  start  the  fundamental  independence  of 
the  colloid  state  of  the  special  chemical  nature  of  the  phases,  for 
to  classify  disperse  systems  according  to  their  degree  of  dispersion 
and  the  state  of  their  phases  makes  use  of  no  chemical  conceptions 
whatever.  On  the  contrary,  it  expressly  departs  from  them.  In 
this  sense,  the  assumption  of  the  universality  of  the  colloid  state 
must  be  termed  the  first  and  most  essential  generalization  that 
may  be  made  regarding  this  class  of  dispersed  systems.  It  was 
therefore  included  in  the  conception  proposed  by  Wolfgang  Ost- 
wald  in  1907  and  developed  independently  of  the  investigations 
of  P.  P.  von  Weimarn.  It  should  be  emphasized,  however,  that 
even  before  the  latter's  investigations,  the  accepted  inductive 
recognition  of  the  essential  independence  of  the  colloid  state  of 
chemical  composition  constituted  one  of  the  essential  steps  which 
made  possible  the  characterization  of  colloids  as  disperse  hetero- 
geneous systems.1 

§18.  Isocolloids 

The  view  that  the  colloid  properties  of  a  substance  represent 
only  properties  of  state  may  perhaps  be  most  strikingly  demon- 
strated by  considering  a  class  of  colloids  in  which  both  disperse 
phase  and  dispersion  means  have  the  same  chemical  composition. 
Among  these  remarkable  colloids  are  found  such  as  consist  of  but 
one  chemical  substance,  in  other  words,  their  disperse  phases  and 
their  dispersion  means  both  consist  of  the  same  substance  but  in 
different  "states"'  We  deal  here  with  systems  which  are  often 

1  See  P.  P.  von  Weimarn  and  Wo.  Ostwald,  Koll.-Zeitschr.,  6,  183  (1910);  P.  P. 
von  Weimarn,  ibid.,  7,  155  (1910). 


DISTRIBUTION    OF    THE    COLLOID    STATE  103 

referred  to  as  "colloid"  or  "colloidally  amorphous,"  but  whose 
place  and  relation  to  the  normal  or  more  common  colloids  has  not 
yet  been  clearly  determined.  We  shall  term  these  structures  in 
which  disperse  phase  and  dispersion  means  are  chemically  iso- 
meric,  isocolloids  (isodispersoids) .  For  those  cases  in  which  a  single 
element  (in  allotropic  forms)  makes  up  the  colloid  system  we  may 
reserve  the  name  allocolloids  (allodispersoids).1 

Examples  of  isodispersoids,  more  particularly  of  isocolloids  are 
common  in  both  inorganic  and  organic  chemistry.  As  mentioned 
in  the  practical  introduction  (p.  i)  liquid  colloids  are  especially 
apt  to  belong  to  the  group  of  liquids  which  behave  " abnormally." 

In  spite  of  agreement  in  elementary  analysis  these  structures 
may,  through  distillation  for  example,  be  divided  into  several 
fractions;  in  other  words,  they  have  no  "constant"  boiling  point. 
Their  internal  friction  often  shows  a  remarkably  high  temperature 
coefficient,  in  other  words,  varies  greatly  with  changing  temperature 
(mixtures  of  fluid  polymers).  Their  so-called  molar  surface 
energy,  that  is,  the  value  V^  .7  (V  =  molar  volume  =  volume  of 
the  gram  molecular  weight;  7  =  the  positive  surface  tension)  is  less 
than  that  of  normal  fluids  (associated  liquids).2  They  can  at  times 
be  separated  through  centrifuging  or  even  filtration,  into  a  solid 
or  semi-solid  phase  and  a  liquid  one.  They  betray  their  physical 
heterogeneity  optically,  for  they  are  turbid,  opalescent,  give  a 
positive  Tyndall  effect,  and  in  the  coarser  dispersoids,  a  "  struc- 
ture" can  be  recognized  microscopically.  Among  these  systems 
are  found  oils,  waxes  and  different  varieties  of  rubber,  the  higher 
fatty  acids,  the  fractions  of  mineral  oils  which  come  off  at  high 
temperatures,3  probably  molten  salts  (which  according  to  R. 
Lorenz4  are  strongly  associated),  and  molten  materials  of  other  com- 
position, as  phosphoric  acid  and  arsenious  acids,  which  on  cooling 
give  rise  to  glacial  forms.  The  name  "glacial"  indicates  that  emul- 
soid  types  of  isocolloids  occur,  and  our  present  knowledge  would 
seem  to  show  that  the  emulsoid  type  is  by  far  the  more  common. 
The  so-called  "meta  forms"  of  molten  materials  (which  are  not 

1  Of  the  available  prefixes,  eu,  hylo,  allo,  auto,  iso,  etc.,  that  of  iso  is  perhaps  the 
best. 

2  For  details  see  the  textbooks  on  Physical  Chemistry,  for  example  Wilh.  Ostwald, 
Grundr.  d.  allgem.  Chemie,  4  Aufl.,  Leipzig,  1909. 

3  For  a  discussion  of  their  colloid  properties  see  D.  Holde,  Koll.-Zeitschr.,  3, 
270  (1908);  Z.  f.  angewandt.  Chem  2138  (1908);  J.  Schneider  and  J.  Just,  Z.  f.  Wis- 
sench.  Mikrosk,  22,  501  (1905). 

4  R.  Lorenz,  Z.  f.  physik.  Chem.,  70,  236  (1910). 


IO4  GENERAL  COLLOID-CHEMISTRY 

to  be  confused  with  the  meta  forms  of  dissolved  materials,  like 
the  different  stannic  acids)  are  particularly  apt  to  produce  jellies 
and  glasses  on  cooling,  as  do  such  typical  emulsoids  as  gelatine  and 
agar.  An  especially  instructive  and  lucid  example  is  the  system 
styrol-metastyrol,  recently  investigated  by  G.  Posnjak.1  Styrol, 
a  hydrocarbon  of  the  composition  CsHg  polymerizes  spontane- 
ously on  standing  into  (C8H8)n  which,  according  to  the  degree  of 
polymerization,  has  a  jelly-like  or  glass-like  consistency  (metas- 
tyrol).  The  polymerization  can  be  followed  quantitatively  by 
measuring  the  internal  friction  which  increases  as  the  polymer- 
ization progresses.  Interestingly  enough  this  particular  example 
shows  what  is  also  true  of  other  substances,  that  light  as  well  as  heat 
favors  polymerization  (photopolymerization).  When  polymeriza- 
tion is  complete  the  product  (metastyrol)  is  hard  and  glassy,  may 
be  pulverized,  etc.  But  the  intermediate  stages  in  the  polymeriza- 
tion are  nothing  more  than  colloid  solutions  of  solid  metastyrol  in 
liquid  styrol.  "If  one  adds  to  pulverized  metastyrol  an  equal 
weight  of  styrol,  the  former  gradually  absorbs  the  latter.  In  the 
process  the  originally  opaque  powder  becomes  translucent  and 
gradually  changes  into  a  homogeneous,  gelatinous  or  jelly-like, 
viscid,  transparent  mass.  If  less  styrol  is  added  to  the  metastyrol , 
say  only  about  a  fourth  as  much  of  the  former  as  of  the  latter,  a 
transparent  mass  results  which  is  not  viscid,  but  glassy"  (G.  Posnjak, 
I.e.,  14).  These  changes  are  entirely  analogous  to  those  observed 
in  the  swelling  of  emulsoids.  That,  on  the  other  hand,  fluid  colloid 
solutions  may  be  obtained  by  employing  an  excess  of  the  liquid 
styrol,  follows  from  the  unlimited  solubility  of  metastyrol  in  styrol2 
as  observed  by  G.  Lemoine.3 

Sulphur4  is  a  typical  allocolloid,  as  are  phosphorus  and  selenium. 

1  G.  Posnjak,  Das  Metastyrol  und  die  beiden  Distyrole,  Diss.,  Leipzig,  1910. 

2  The  different  polymerization  or  dispersion  states  of  styrol  are  preserved  even 
when  dissolved  in  a  chemically  heterogeneous  solvent  such  as  carbon  tetrachloride. 
This  is  proved  not  only  by  the  fact  that,  like  a  typical  colloid,  metastyrol  when  in 
solution,  causes  no  rise  in  the  boiling  point,  but  also  by  the  different  reactivity 
observed  in  the  different  stages  of  polymerization,  When  a  drop  of  permanganate 
solution,  made  alkaline  with  sodium  hydroxide,  is  added  to  solutions  of  liquid  styrol, 
gelatinous  styrol  and  solid  metastyrol,  having  the  same  percentage  concentrations,  it  is 
decolorized  respectively  in  10  seconds,  in  40  seconds  and  650  seconds  (G.  Posnjak, 
l.c.,  16). 

8  G.  Lemoine,  Compt.  rend.,  125,  530  (1897);  129,  719  (1899). 

4  I  have  previously  pointed  out  the  great  interest  attached  to  a  considera- 
tion of  the  allotropic  forms  of  sulphur  from  a  colloid-chemical  standpoint,  Koll.- 
Zeitschr,  7,  172  (1910). 


DISTRIBUTION   OF   THE    COLLOID    STATE  10$ 

As  is  well  known,  the  soft,  plastic,  translucent  to  transparent1  form 
of  sulphur  obtained  by  cooling  it  rapidly  has  long  been  called 
colloid  or  "colloidally  amorphous"  sulphur.  As  the  result  of 
many  investigations,  we  have  been  forced  to  assume  the  existence 
of  several  more  so-called  allotropic  modifications  more  especially 
of  two  liquid  forms  of  sulphur,  designated  as  Sx  and  SM.  When 
pure  sulphur  is  heated  one  obtains  the  modification  Sx  which  is  a 
bright  yellow,  labile  liquid,  from  the  time  the  sulphur  first  melts 
up  to  i6o°C.  If  the  temperature  is  further  raised  the  system 
again  becomes  viscid  and  its  surface  tension  again  increases. 
If  this  melted  sulphur  is  cooled  to  100°  then  as  Malus,  F.  Hoffmann 
and  R.  Rothe,  A.  Smiths,  etc.,2  have  shown  a  second  liquid  sulphur 
phase,  SM  makes  its  appearance  which  gives  rise  to  the  so-called 
"insoluble"  sulphur  when  the  sulphur  solidifies.  Between  160° 
and  about  270°  in  the  range  of  the  physical  irregularities  men- 
tioned above,  the  system  is  therefore  allodispersed  and  it  seems 
logical  to  assume  that  at  certain  temperatures,  a  colloid  condition 
is  traversed.  The  physical  characteristics  observed  in  the  be- 
havior of  sulphur  remind  us  in  many  respects  of  the  behavior  of 
emulsoid  colloids.3 

1  Concerning  perfectly  "transparent"  sulphur  see  P.  P.  von  Weimarn,   Koll.- 
Zeitschr.,  6,  250  (1909). 

2  An  extended  discussion  of  the  work  done  on  sulphur  up  to  1902  may  be  found  in 
Wilh.  Ostwald,  Lehrb.  d.  allg.  Chem.  2,  Aufl.  II,  2,  449.   See  also  the  recent  extensive 
papers  of  A.  Smiths  and  his  coworkers,  Zeitschr.  f.  physik.  Chem.,  42,  469  (1903); 
52,  602  (1905);  54,  276  (1906);  57,  685,  692  (1907);  61,  200  (1907);  F.  Hoffmann 
and  R.  Rothe,  ibid.,  55, 113  (1906);  59,  448  (1907);  H.  R.  Kruyt,  ibid.,  64,  513  (1908) 
where  extensive  references  to  the  literature  may  be  found;  65,  486  (1909),  etc. 

3 1  purpose  publishing  details  regarding  these  analogies  elsewhere.  Here  I  would 
only  point  out  that  sulphur  melts  bring  to  mind  the  critical  fluid  mixtures  such  as 
those  of  butyric  acid  and  water,  a  fact  which  seems  not  previously  to  have  been 
noted.  Thus  the  viscosity  curves  in  the  temperature  ranges  where  an  anomalous 
behavior  is  noted  [see  L.  Rotinjanz,  Z.  f.  physik.  Chem,,  62,  609  (1908)]  are  iden- 
tical in  form  with  the  corresponding  friction  curves  of  aqueous  critical  fluid  mixtures 
[J.  Friedlander,  ibid.,  38,  430  (1901)].  Attention  should  also  be  called  to  the  non- 
conclusiveness  of  the  view  that  both  fluid  phases  "cannot"  exist  in  equilibrium 
above  and  below  the  "transition"  point  (about  160°)  because  this  would  contradict 
the  phase  rule  [see  especially  H.  R.  Kruyt,  I.e.].  As  a  matter  of  fact  the  apparently 
unlimited  stability  of  critical  fluid  mixtures  as  proved  by  Friedlander's  careful 
studies  seems  to  indicate  that  "real"  equilibria  and  not  simply  "dynamic"  retarda- 
tions exist  in  the  case  of  sulphur  also.  Those  investigators,  who  assert  that  the 
existence  of  true  equilibria  would  contradict  the  phase  rule,  forget  that  this  rule 
holds  only  if  the  nature  of  the  phases  is  the  same  throughout  their  entire  mass,  or,  to 
quote  Gibbs  himself,  only  "if  the  changes  of  the  shares  of  energy  and  entropy  which 
arise  from  the  surfaces  between  the  heterogeneous  masses,  are  so  small  in  comparison 
with  those  which  arise  from  these  masses  themselves  that  they  are  negligible.  In 
other  words,  we  will  exclude  any  consideration  of  the  effects  of  capillarity."  [Thermo- 
dynamische  Studien,  Leipzig,  1892,  75;  see  especially  pp.  89  and  90  where  the 
reasons  for  the  limitation  mentioned  above  are  given  in  greater  detail.]  But  this 


106  GENERAL  COLLOID-CHEMISTRY 

As  shown  by  the  photographic  investigations  of  O.  Biitschli1 
and  A.  Wigand,2  highly  dispersed  solid  systems  of  sulphur  may 
also  be  produced.  It  would  be  of  interest  to  extend  these  investi- 
gations into  the  ultramicroscopic  realm  of  the  colloids.  This  has 
been  done  by  H.  Siedentopf 3  for  phosphorus  which  shows  analogous 
separation  phenomena  under  the  influence  of  intense  light.4 
White  phosphorus,  as  is  well  known,  is  converted  into  red,  by 
light.  The  intense  light  of  a  so-called  "  cardioid  "  ultramicroscope 
accomplishes  this  in  a  few  seconds.  Ultra-microscopically,  a 
solidified  drop  of  white  phosphorus  at  first  appears  " empty,"  but 
almost  instantly,  upon  illumination,  white  submicrons  appear. 
These  grow  and  combine  through  tendril-like  extensions  into  a 
kind  of  network,  and  finally  become  red.  According  to  these 
investigations  white  phosphorus,  with  a  trace  of  red  phosphorus 
undoubtedly  represents  a  solid,  at  first  perhaps,  semi-solid 
allocolloid. 

§19.  Multiplicity  of  the  Colloid  State  of  One  and  the  Same  Sub- 
stance. 'Example:  Colloid  Ice 

As  a  further  evidence  of  the  fundamental  independence,  of  the 
colloid  state  of  the  chemical  character  of  the  substance,  and  as  an 

assumption  of  Gibbs  as  used  in  his  general  considerations  of  the  phase  rule  is,  of 
course,  not  true  as  I  have  repeatedly  emphasized,  and  as  Gibbs  himself  has  said, 
when  we  deal  with  disperse  and  more  especially,  colloid  systems.  Gibbs 
started  with  the  assumption  that  we  could  neglect  that  part  of  the  energy,  etc.,  which 
originates  in  the  surfaces  between  the  heterogeneous  masses.  Such  an  assumption  is 
fully  justified  in  many  cases  and  for  many  purposes,  or  whenever  the  masses  are 
large;  but  when  the  masses  are  formed  in  or  between  materials  of  different  nature  or 
state,  or  are  at  the  instant  of  formation  infinitely  small,  such  an  assumption  becomes 
unreliable,  for  now  the  surfaces  become  infinitely  large  as  compared  with  the  masses. 
In  answer  to  the  question  as  to  whether  the  phase  rule  holds  for  colloids,  we  may  say, 
that  a  phase  rule  which  recognizes  only  concentration,  pressure  and  temperature  as 
variables,  is  not  going  to  be  valid,  but  an  elaborated  one  in  which  the  degree  of  dis- 
persion of  the  system  is  added  will  be.  The  same  is  true  in  dealing  with  electrically 
charged  disperse  phases,  in  which  account  must  be  taken  of  the  influence  of  electrical 
energy  upon  the  equilibrium  of  the  system;  for  as  is  well  known,  the  latter  is  not,  or  at 
least  not  always,  a  function  of  the  mass  of  the  charged  phase.  Regarding  the  val- 
idity of  the  phase  rule  in  colloid  systems  see  J.  M.  van  Bemmelen,  Die  Absorption, 
Ges.  Abhandlgn.,  347;  Dresden,  1910;  A.  Mittasch,  Zeitschr.  f.  physik.  Chem.,34, 495 
(1900);  G.  Galeotti,  ibid. ,54,  727  (1905);  P.  Pawlow,  ibid.,fst  48  (1910).  In  the  last- 
named  paper  which  appeared  while  these  paragraphs  were  in  press,  the  phase  rule  is 
broadened  as  suggested  above. 

1  O.  Butschli,  Untersuchungen  iiber  Strukturen,  Leipzig,  1898;  Die  Mikrostruk- 
turen  des  estarrten  Schwefels,  Leipzig,  1900. 

2  A.  Wigand,  Zeitschr.  f.  physik.  Chem.,  72,  752  (1910). 

3  H.  Siedentopf,  Ber.  d.  dtsch.  chem.  Ges.,  43,  692  (1910). 

4  As  is  well  known,  sulphur  also  suffers  an  allotropic  change  under  the  influence 
of  light  (Daguin,  Lallemand,  Berthelot,  Rankin,  etc.;  see  H.  R.  Kruyt,  I.e.,  543 
(1908). 


DISTRIBUTION   OF   THE    COLLOID    STATE  1 07 

example  of  the  multiplicity  of  colloid  phenomena  that  may  be 
exhibited  by  one  and  the  same  substance,  we  shall  consider  the 
possible  colloid  forms  of  water,  and  those  which  have  actually 
been  prepared.  As  follows  from  the  chapters  on  the  influence  of 
the  degree  of  dispersion  and  of  the  type  of  the  dispersed  material 
upon  the  colloid  state,  the  same  chemical  compound  may  give 
rise  not  only  to  one,  but  to  a  large  number  of  colloid  systems. 
If  our  view  is  correct,  that  only  a  certain  intermediate  degree  of 
dispersion  gives  the  property  of  a  colloid,  then  it  should  be  a 
matter  of  indifference,  in  the  case  of  water,  with  which  form  we 
begin.  From  it  we  should  be  able  to  prepare  all  its  various  colloid 
states.  That  such  is  possible  seems  proved  by  recent  investiga- 
tions (G.  Quincke,1  P.  P.  von  Weimarn  and  Wo.  Ostwald,2  H. 
Schade3). 

1.  Isocolloids  of  H2O. — Let  us  commence  with  the  isocolloid 
state.     The  following  possibilities  exist : 

(a)  Solid  ice   (dispersion    means)  +  water -vapor    bubbles    of 
colloid  dimensions  (disperse  phase).     Coarsely  dispersed  systems 
of  this  character,  as  the  turbid  or  milky- white  ice   (milk    ice) 
produced    by    refrigerating    machines,   are    well    known.     It    is 
probable  that  a  more  exact  study  of  the  relation  between  degree 
of  turbidity  or  diameter  of  the  bubbles  and  the  conditions  pre- 
vailing during  preparation  of  the  ice  will  acquaint  us  with  systems 
showing  a  colloid  degree  of  dispersion. 

(b)  Solid  ice  (dispersion  medium)  +  liquid  droplets  of  colloid 
diameter    (disperse   phase).     Considerations   analogous  to  those 
discussed  in   the  preceding  paragraph  hold  for   this   case.     G. 
Quincke  (I.e.)'  regards  it  as  the  most  general  structure  of  solid, 
" amorphous"  ice. 

(c)  Solid  ice    (dispersion   medium)  +  solid   ice   of   a   colloid 
degree  of  dispersion  (disperse  phase).     That  such  systems  exist 
is  proved  by  the  investigations  of  G.  Tammann,4  who,  as  a  matter 
of  fact,  recognizes  three  or  four  different  modifications  of  solid 
ice.     According  to  G.  Quincke  (I.e.)  the  forms  of  ice  described  in 

1  G.  Quincke,  Drude's  Ann.  d.  Physik.,  18,  n  (1905). 

2  P.  P.  von  Weimarn  and  Wo.  Ostwald,  Koll.-Zeitschr.,  6,  181  (1910). 

3  H.  Schade,  Koll.-Zeitschr.,  7,  26  (1910).     See  also  the  extended  discussion  of  this 
theme  in  Trans.  Faraday  Soc.,  6,  71-129  (1910). 

4  G.  Tammann,  Zeitschr.  f.  physik.  Chem.,  69,  569  (1910);  72,  609  (1910);  Zeit- 
schr.  f.  anorgan.  Chem.,  63,  283  (1909)  where  may  be  found  further  references  to  the 
literature  on  the  different  modifications  of  ice. 


108  GENERAL  COLLOID-CHEMISTRY 

the  previous  paragraphs,  may,  by  lowering  the  temperature,  by 
freezing  at  different  rates,  etc.,  be  converted  into  the  system  dis- 
cussed here. 

(d)  Fluid  water  (dispersion  medium)  +  water-vapor  bubbles  of 
colloid  dimensions  (disperse  phase).     Highly  dispersed  systems 
of  this   type   are  represented  by  the   chemically  homogeneous 
fluids    near    their    critical    vaporization    temperatures.     Their 
fine  turbidity  or  strong  opalescence,  their  varying  density,  etc., 
etc.,  betray  their  colloid  character.1 

(e)  Fluid  water  (dispersion  medium)  +  fluid  water  droplets 
of  colloid  dimensions  (disperse  phase).     As  is  well  known,  water 
belongs  to  the  strongly  associated  liquids,  so  that  the  formation 
of  complexes   and   of  polymeric   molecules    (polyhydrols)   more 
especially  at  lower  temperatures  has  often  been  called  upon  to 
explain  its  anomalies  in  behavior.2     Since  mo'dern  theoreticians 
hold  that  this  polymerization  may  be  so  great  that  they  speak 
of  the  formation  of  "droplets"  of  the  polymer  in  the  non-asso- 
ciated liquid,  it  is  easy  for  us  to  assume  that  colloid  degrees  of 
dispersion  may  be  reached.     H.  Schade  (I.e.)  has,  as  a  matter 
of  fact,  developed  such  a  colloid-chemical  theory  of   the   Con- 
stitution of  water,   according  to  which,  its  behavior  in  many 
directions  is  shown   to  be  analogous  to  that  of  well-recognized 
colloids.     The  observation  of  P.  P.  von  Weimarn  (I.e.)  that  sud- 
denly cooled  water  (induced  by  mixing  with  liquid  air)  is  at  first 
soft  and  viscid,  brings  to  mind  the  behavior  of  melted  sulphur, 
when  this  is  cooled  to  ioo°C. 

(/)  Fluid  water  (dispersion  medium)  +  colloidally  dispersed 
solid  ice  (disperse  phase) .  It  cannot  be  definitely  decided  whether 
highly  dispersed  examples  of  this  type  really  exist.  Snow  in 
water  would  represent  a  coarsely  dispersed  system  of  this  type. 

(g)  Water  vapor  (dispersion  medium)  +  water  droplets  of 
colloid  dimensions  (disperse  phase).  Examples  of  this  are  found 
among  the  critical  phenomena  as  in  the  liquefaction  of  water 
vapor.  Such  structures,  usually  called  mists  or  fogs,  play  a 
great  part  in  cosmic  physics,  where  they  are  known  as  clouds,  or 
when  in  a  coagulated  state,  as  rain  (P.  Pawlow).3 

1  See,  for  example:  J.  P.  Kuenen,  Die  Zustandsgleichung,  34.   Braunschweig,  1907. 

2  See  H.  Schade  for  references  to  the  literature  on  this  subject. 

8  P.  Pawlow,  Koll.-Zeitschr.,  8  (1911).  I  also  pointed  out  that  these  cosmic 
structures  are  examples  of  dispersed  or  colloid  systems  [Koll.-Zeitschr.,  I,  291,  331 
(1907),  and  in  the  first  edition  of  this  book,  96  (1909)]. 


DISTRIBUTION   OF    THE    COLLOID    STATE  109 

(ti)  Water  vapor  (dispersion  medium)  +  colloidally  dispersed 
ice  (disperse  phase).  Such  systems  are  obtained  as  fine  snows 
when  fogs  are  rapidly  cooled  below  their  freezing  point.  Their 
optical  properties,  which  remind  us  of  the  Tyndall  effect  of  col- 
loid systems,  are  observable  on  winter  nights,  when  the  moon  has 
a  halo.  The  fact  that  in  winter  we  deal  with  fine  particles  of 
solid  ice  explains  why  the  halo  is  then  more  marked  than  in  sum- 
mer when  the  mists  are  liquid  in  character. 

2.  Chemically  Heterogeneous  H-,0  Colloids. — When  only  the 
disperse  phase  consists  of  water,  while  the  dispersion  means  is 
another  substance,  then  as  in  the  case  of  the  water-isocolloids 
eight  colloid  systems  are  possible.  The  water  must  of  course  be 
insoluble  in  such  systems,  or  at  least  but  slightly  soluble,  otherwise 
molecular  dispersoids  result.  It  is  an  easy  matter  to  take  up  these 
eight  possibilities  and  parallel  them  with  what  was  said  before, 
and  to  find  examples  for  the  different  types.  Of  especial  interest 
are  the  ice  colloids  of  the  composition  liquid  +  gas,  liquid  + 
liquid  and  liquid  +  solid,  in  other  words,  the  aqueous  colloid 
"foams,"  the  aqueous  emulsoids  and  the  ice  suspensoids.  In  the 
known  examples  of  the  first  two  of  these  classes,  we  seem  again  to 
be  dealing  as  a  rule,  with  complex  dispersoids,  in  that  the  gas 
bubbles  obtained,  say,  by  sudden  diminution  of  pressure  in  an 
alcohol-water  mixture  (champagne  or  charged  water  constitute 
crude  examples),  usually  consist  of  a  mixture  of  several  gases  (water 
vapor,  alcohol  vapor,  air,  carbon  dioxide,  etc).  More  decisive 
experiments  could,  of  course,  be  arranged.1  The  commonest 
illustrations  of  highly  dispersed  water  are  also  found  in  complex 
systems,  as  in  phenol-water,  mineral  oil-water,  etc.  Such  sys- 
tems, as  a  rule,  are  rather  unstable,  their  instability  increasing 
with  the  purity  of  the  liquids  employed.  The  stability  may 
be  greatly  increased  by  adding  substances  like  saponin,  soap, 
gelatine,  etc. 

Finally,  highly  dispersed  colloid  systems  of  the  composition 
liquid  +  solid,  in  other  words,  ice  suspensions  and  ice  suspensoids, 
have  recently  been  prepared.  Such  systems  are  obtained  when 
organic  liquids  in  which  water  is  only  slightly  soluble  in  molecular 

1  For  some  observations  on  highly  dispersed  aqueous  foams  see  Wo.  Ostwald, 
Koll.-Zeitschr.,  I,  333  (1907). 


IIO  GENERAL  COLLOID-CHEMISTRY 

form  are  rapidly  cooled  while  thus  saturated  with  water.  Ether, 
xylol  and  especially  chloroform  have  been  found  suitable  for  this 
purpose.  Depending  upon  the  amounts  of  water  dissolved,  the 
rate  of  cooling,  etc.,  systems  of  varying  degrees  of  dispersion  are 
produced,  the  turbidity  of  which  is  the  less,  or  the  opalescence 
(yellowish-blue)  of  which  is  the  greater,  the  more  highly  dispersed 
the  ice  phase  which  separates  out.  Under  favorable  conditions, 
these  mixtures  coagulate  spontaneously  after  standing  some 
40  minutes,1  in  the  form  of  snow-white  flakes  or  " cream,"  which 
rise  to  the  surface  of  the  liquid.  The  addition  of  certain  sub- 
stances like  resins,  the  salts  of  fatty  acids,  etc.,  greatly  increases 
both  stability  and  degree  of  dispersion.  By  rapidly  chilling  chloro- 
form saturated  with  water  to  —  30°,  ice  dispersoidsmaybe  obtained 
which  pass  through  filter  paper  (Schleicher  and  Schiill,  602,  extra 
hard).  This  indicates  that  the  ice  particles  are  less  than  i/* 
in  diameter,  in  other  words,  that  they  are  already  within  the  colloid 
range  of  dispersion. 

The  possible  colloid  forms  of  water  are  not  exhausted  by  these 
sixteen  types.  It  was  pointed  out  on  pp.  35,  44,  etc.,  that 
colloid  systems  assume  different  properties  with  differences  in  the 
relative  proportions  of  disperse  phase  and  dispersion  means. 
This  is  especially  true  when  very  dilute  colloids  are  compared  with 
concentrated  ones.  Thus  while  many  colloids  with  a  small 
content  of  disperse  phase  show  suspensoid  properties,  the  more 
concentrated  systems  often  have  emulsoid  properties,  they  are,  in 
other  words,  jelly  or  glass-like.  One  suspects  that  he  is  here 
dealing  with  a  general  law  and  so  looks  for  such  in  the  extreme 
concentrations  of  ice  colloids.  Thus  J.  Alexander2  has  shown  that 
"ice  creams"  consist  of  numerous  highly  dispersed  ice  crystals 
in  the  dispersion  medium,  cream,  to  which  a  trace  of  gelatine  is 
often  added.  The  bulk  of  the  ice  crystals  is  so  great  as  compared 
with  that  of  the  dispersion  means,  that  as  in  the  case  of  fine,  moist 
sand,  the  dispersion  medium  covers  the  solid  dispersed  particles 
with  a  thin  but  coherent  envelope. 

So  far  as  structures  of  the  composition  liquid  +  liquid  are 
concerned,  we  may  direct  attention  to  the  experiments  of  Wa. 
Ostwald3  which  indicate  the  possible  existence  of  emulsions  in 

1  See  P.  P.  von  Weimarn  and  Wo.  Ostwald,  Lc. 

2  J.  Alexander,  Koll.-Zeitschr.,  4,  168  (1909);  5,  101  (1909). 

3  Wa.  Ostwald,  Koll.-Zeitschr.,  6,  103  (1910). 


DISTRIBUTION   OF    THE    COLLOID    STATE  III 

which  the  disperse  (aqueous)  phase  is  present  in  excess.  Foams 
may  also  occur  in  these  two  modifications.  We  may  have  fluid 
structures  made  turbid  by  minute  bubbles  (see  above)  or  "stiff 
foams"  consisting  chiefly  of  vapor,  held  together  by  a  small 
amount  of  fluid  dispersion  means,  as  in  well-whipped  saponin, 
albumin,  beer,  etc.  It  is  characteristic  of  most  of  these  systems 
that  they  are  fairly  stable  only  when  certain  third  substances  are 
present. 

In  concluding  these  paragraphs  it  should  be  noted  that  even 
with  this  long  list  all  the  theoretical  possibilities  are  not  yet 
exhausted.  For  example,  there  are  several  solid  modifications  of 
ice  known,  any  of  which  may  appear,  theoretically  at  least,  as 
disperse  phase  or  dispersion  means;  and  when  we  consider  com- 
pounds or  elements  whose  polymorphism  is  still  greater,  as  in 
the  numerous  solid,  liquid  and  gaseous  modifications  of  sulphur, 
the  number  of  disperse  and  colloid  states  of  one  and  the  same 
substance,  which  are  theoretically  possible,  becomes  almost 
limitless. 

§20.  The  Concept  of  Colloid-chemistry 

The  most  important  deduction  from  the  previous  para- 
graphs is  that  it  is  no  longer  appropriate  to  contrast  colloid 
substances  with  crystalloid  substances  as  though  the  condition 
were  dependent  upon  the  specific  chemical  properties  of  the 
material.  Colloid-chemistry  is  not  the  study  of  colloid  materials 
but  that  of  the  colloid  state  of  materials  (Wo.  Ostwald,  1908). 
" Colloid"  is  not  a  chemical  entity  like  salt,  acid,  base,  oxidizing 
or  reducing  agent,  but  is  expressive  of  certain  physical  elements 
like  mechanical  heterogeneity.  The  concept  " colloid"  does  not 
even  correspond  to  that  of  "  precipitate "  since  only  special 
forms  of  precipitates  may  be  termed  " colloid."  Nor  may  "col- 
loid" substances  be  discussed  as  we  discuss  "radio-active"  sub- 
stances, for  radio-active  properties  are  more  closely  associated  with 
certain  chemical  compounds  showing  definite  properties  (high 
atomic  weight,  etc.),  than  are  the  colloid.  Like  considerations 
hold  when  we  try  to  parallel  the  colloid  condition  with  the  "liquid 
crystalline,"  though  as  our  knowledge  has  increased  we  have 
found  the  latter  state  less  and  less  directly  connected  with  definite 


112  GENERAL  COLLOID-CHEMISTRY 

chemical  compounds.1  In  the  same  sense  " colloid  phenomena" 
are  not  to  be  regarded  as  due  to  the  properties  of  colloid  materials 
but  rather  as  characteristic  of  any  material  observed  in  the  colloid 
state.  The  difference  between  these  two  definitions  will  perhaps 
be  clearer  if  we  compare  colloid-chemistry  with  thermo-chemistry. 
Just  as  the  latter  is  not  a  study  of  "warm"  and  "cold"  materials, 
but  a  study  of  the  thermal  condition  of  the  material  and  its  changes, 
so  colloid-chemistry  is  not  a  description  of  individual  colloid 
materials  but  treats  of  the  properties  of  which  colloid  systems 
are  but  examples.  Colloid-chemistry  deals  with  the  relations  of 
the  surface  energies  to  other  kinds  of  energy  as  shown  in  an  '•espe- 
cially characteristic  way  in  dispersed  heterogeneous  systems.  Thus 
viewed,  colloid-chemistry  appears  as  a  branch  of  physical  chem- 
istry coordinated  with  electro-,  thermo-,  photo-,  radio-chemistry, 
etc.,  in  other  words,  with  sciences  which  also  treat  of  the  relations 
of  one  kind  of  energy  to  others.  Attempts  have  been  made  to 
express  this  by  calling  it  capillary  chemistry  (Freundlich),  strato- 
chemistry  (Drucker),  micro-chemistry  (Wilh.  Ostwald),  etc.  Since 
philological  and  practical  objections  may  be  raised  against  most 
of  these  terms  the  historically  justified  and  useful  one  of  "colloid- 
chemistry"  will  be  retained  in  this  work. 

1  We  need  but  recall  the  "inorganic"  liquid  crystals  recently  discovered.     See 
H.  Stoltzenberg  and  M.  E.  Huth,  Z.  f.  physik.  Chem.,  71,  641  (1910),  etc. 


PART  II 

SPECIAL  COLLOID-CHEMISTRY 

A.  THE  GENERAL  .PHYSICO-CHEMICAL 
PROPERTIES  OF  COLLOIDS 


CHAPTER  V 

MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 

I.  RELATIONS  OF  VOLUME  AND  MASS  IN  COLLOIDS 

§21.  Volume  and  Density  Relations  in  Colloids 

i.  Volume  Relations  of  Colloid  Systems. — Compressibility  — 
In  one  of  the  previous  sections  (§14)  reference  has  been  made 
to  the  fact  that  the  surface  between  two  phases  (for  example 
the  surface  between  a  liquid  and  a  gaseous  phase)  has  properties 
distinctively  its  own.  *These  peculiar  surface  properties  do  not 
extend  much  below  the  surface,  not  deeper  than  the  "  sphere  of 
molecular  attraction."  But  in  spite  of  the  thinness  of  this  layer 
it  does  not  resemble  a  mere  shell,  in  other  words,  it  is  not  sepa- 
rated from  the  rest  of  the  phase  by  a  sharp  line.  Mathematical 
considerations1  would  seem  to  indicate  that  there  is  a  continuous 
change  of  properties  within  this  layer,  extending  asymptotically 
into  the  depth  of  the  phase.  In  addition  to  the  fact  that  the 
surface  of  contact  is  the  seat  of  surface  energy  we  must  also 
assume  that  its  hydrostatic  pressure  and  the  related  properties 
of  volume  and  density  differ  from  these  same  properties  as  ex- 
hibited by  the  interior  of  the  phase.  We  have  also  pointed  out 
how  in  coarsely  disperse  systems  these  differences  can  be  demon- 
strated only  with  great  difficulty.  But  the  slight  differences  ob- 
served in  coarsely  disperse  systems  add  up  to  considerable 
values  in  systems  which  have  great  areas  of  " surface  contact." 
Especially  is  this  true  in  the  disperse  heterogeneous  systems  where 
a  great  specific  surface  is  found  with  great  absolute  surface.  The 
relation  between  specific  surface  and  volume  is  established  by 
the  capillary  or  curvature  pressure  (Krummungsdruck). 

Consideration  of  the  progressive  change  in  the  properties  of 
the  surface  and  the  effects  of  capillary  pressure  lead  to  the  con- 
clusion that  the  volume  of  the  dispersoid  need  not  be  equal  to  the 
arithmetic  mean  of  the  dispersion  means  plus  the  disperse  phase. 

1  See  for  example  van  der  Waals  and  Kohnstamm,  Lehrb.  der  Thermo-dynamik 
I,  64,  as  well  as  Hulshof  from  whose  works  we  have  quoted. 


Il6  SPECIAL  COLLOID-CHEMISTRY 

The  observed  values  are  usually  less  than  the  arithmetic  mean. 
This  is  in  harmony  with  the  assumption  that  the  effect  of  positive 
capillary  pressure  generally  outweighs  that  of  the  negative.1 
The  amount  of  this  contraction  besides  being  a  function  of  the 
positive  capillary  pressure  is  evidently  a  function  of  the  com- 
pressibility or  expansibility  of  the  two  phases  in  the  sense  that  a 
great  coefficient  of  compression  or  expansion  will  favor  change  in 
volume. 

It  seems  important,  therefore,  to  consider  the  compression 
coefficients  of  colloid  solutions.  Theoretically,  three  compressi- 
bilities must  be  considered  in  a  colloid  (or  in  any  dispersoid) :  the 
compressibility  of  the  dispersion  means,  the  compressibility  of  the 
disperse  phase,  the  compressibility  of  the  system  as  a  whole. 
The  first  and  third  of  these  can  be  measured  by  physical  methods, 
the  value  of  the  second  may  be  calculated  from  the  other  two. 
Of  greatest  immediate  interest  is  a  comparison  of  the  compressi- 
bility of  a  colloid  solution  with  that  of  the  pure  dispersion  means. 
One  anticipates  that  the  compressibility  of  the  colloid  solution  will 
be  smaller  than  that  of  the  pure  dispersion  means,  and  that  it  will 
decrease  as  the  concentration  of  the  colloid  increases.  That  these 
relations  will  be  more  complicated  in  emulsoids  than  in  suspensoids 
is  also  to  be  expected.  The  curve  expressing  the  relation  between 
concentration  and  compression  of  emulsoids  will  probably  not  be  a 
straight  line  as  in  suspensoids.  The  fact  that  it  is  not  a  straight 
line  in  molecular-  and  ionic-  disperse  systems  already  indicates  this; 
for  as  the  investigations  of  Rontgen  and  Schneider,2  H.  Gilbaut3 
and  others  have  shown,  this  function  approximates  a  hyperbolic 
curve  diverging  more  and  more  from  a  straight  line  with  increasing 
concentration  (see  Fig.  20).  Fig.  17  shows  the  relation  between 
compressibility  and  concentration  of  NaCl  according  to  the 
experiments  of  H.  Gilbaut.  A  similar  behavior  is  to  be  expected 
for  the  emulsoids  because  of  their  close  relation  to  the  molecular 
dispersoids. 

One  would  also  expect  that  electrically  charged  or  ionized 
colloids  would  decrease  the  compressibility  of  the  pure  dispersion 
means  more  than  such  not  so  charged,  for  according  to  Guinchant4 

1  See  p.  91. 

2  W.  Rontgen  and  Schneider,  Wiedemann's  Ann.  d.  Physik.,  29,  165  (1886). 

3  H.  Gilbaut,  Z.  f.  physik.  Chem.,  24,  385  (1897). 

4  Guinchant:  Compt.  rend.,  132,  469  (1901). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


117 


the  compressibility  of  water  is  less  reduced  through  addition  of  a 
non-electrolyte  than  of  an  electrolyte. 

Of  the  available  determinations  on  colloids  those  of  G.  de  Metz1 
merit  special  attention.  Table  3  gives  a  selection  from  his 
careful  measurements.  For  comparison  the  values  of  water  and 
some  other  non-colloid  solutions  are  appended.  The  table  shows 
that  at  least  as  far  as  the  emulsoids  which  have  thus  far  been  studied 
are  concerned  the  compressibility  of  colloids  is  not  essentially 
different  from  that  of  other  non-colloid  liquids.  The  compressi- 


ConcenlraHon *~ 

FIG.  17. — Compressibility  of  NaCl  solutions  in  concentrations  varying  between  o 
and  26.22  per  cent.     (According  to  H.  Gilbaut.) 


bility  of  collodion  is  twice  that  of  water,  but  crystallized  benzene 
has  also  an  abnormally  high  coefficient.  In  the  case  of  hydrosols 
we  find,  with  the  exception  of  setting  gelatine,  that  the  coeffi- 
cient of  compressibility  is  lower  than  that  of  the  pure  dispersion 
means  which  again  is  analogous  to  the  behavior  of  molecular  dis- 
persoids.  This  also  seems  to  apply  to  benzene-sols,  as  comparison 
of  the  values  for  pure  benzene  and  for  a  solution  of  Canada  balsam 
in  benzene  indicates. 

1  G.  de  Metz,  Wiedemann's  Ann.  d.  Physik.,  35,  497  (1888);  see  also  G. 
Quincke,  Ibid.,  19,  401  (1883);  E.  H.  Amagat,  Ann.  chim.  et  phys.  (5),  n,  535 
(1877)- 


n8 


SPECIAL  COLLOID-CHEMISTRY 


TABLE  3. — COMPRESSION  COEFFICIENTS  OF  COLLOID  SOLUTIONS 
(From  G.  de  Metz) 


Substance 

Specific  gravity 

Compression  coefficient 
(absolute)  k.io-« 

Non-gelatinous  glue  

1.053    (l4-8°) 

44    337    (l2    18°) 

Gum  arable  in  water  
Gelatinizing  glue*  
Canada  balsam  in  benzol  .... 
Duplex  collodion.  ...        .... 

1.041    (14-0°) 

1.005  (18.2°; 

0.950   (15.0°) 
o  807  (15  o°) 

44-593    (14-84°) 
48.388    (11.67°) 
57.205    (14-90°) 
07    433    (lA   8<?°) 

1.345  (14.5°) 

25.509    (14-64°) 

Water          

i           (15  0°) 

47.430    (l2    58°) 

Sugar  in  water 

I    3  SO    (i*    er°) 

2O    827    (14    80°) 

Metaphosphoric  acid  in  water 
Glycerine 

1-545    (I3.50) 

i   245  (16  5°) 

19.663    (14.68°) 
22    128    (14    92°) 

Benzene  (crystallized)  

0.882  (18.2°) 

74.609    (    4-77°) 

Liquid  paraffine  

0.860  (17.0°) 

62.865    (14.84°) 

*  The  compressibility  changes  with  time. 

The  behavior  of  gelatinizing  glue  is  particularly  interesting,  for 
its  compressibility  decreases  with  time.  According  to  G.  de  Metz, 
a  2  per  cent,  solution  showed  when  first  measured  a  compressi- 
bility of  51.42  (X  io~6);  three  hours  later  this  decreased  to  49.73, 
and  ten  days  later  to  48.44.  In  other  words,  as  gelation  pro- 
ceeded the  compressibility  approached  more  and  more  the  value 
of  the  pure  dispersion  means  (water).  This  observation  is  of 
interest  in  its  application  to  the  theory  of  gelation  in  emulsoids. 

Other  compressibility  determinations  have  been  made  by  C. 
Barus.1  Barus  measured  the  height  of  liquids  in  capillary  glass 
tubes  at  various  pressures.  The  compressibility  is  equal  to  the 
decrease  in  height  divided  by  the  total  height  of  the  liquid  in 

the  capillary  tubes  (  =y  ).     Table  4  gives  some  of  his  results. 

These  figures2  show  that  compressibility  differences  between 
colloids  and  their  pure  dispersion  means  are  slight,  especially  when 
compared  with  the  corresponding  variations  observed  in  molecular 

1  C.  Barus,  Silliman's.  Amer.  Journ.  Science  (4),  6,  285,  (1898);  (3),  Ibid.,  41,  no 
(1891),  Compressibility  of  Glass. 

2  For  further  tables,  for  example,  on  the  compressibility  of  coagulated  albumin, 
etc.,  see  the  original  work  of  C.  Barus. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 

TABLE  4. — COMPRESSIBILITY  OF  COLLOID  SOLUTIONS 
(After  C.  Bams) 


IIQ 


r  =  23°; 

Water. 

L  =  17.4  cc.;  p  =  Atm. 

Natural  egg  -albumin 
T  =  22°;  L  =  12.3  cc.;  p  =  Atm. 

p 

1 

L 

y 

1 

L 

0 

o  .  oooo 

O 

o  .  oooo 

83 

0.0037 

81 

o  .  0039 

1  60 

0.0075 

128 

0.0061 

226 

0.0108 

191 

0.0093 

T  =  100° 

Water. 

;  L  =  1  8.  i  cc.;  p  =  Atm. 

Gelatine  10  per  cent. 

T  =  100°;  L  =  21.1  cc.;  p  =  Atm. 

P 

1 

L 

P 

1 
L 

o 

0  .  OOOO 

0 

o  .  oooo 

83 

o  .  0046 

116 

0.0058 

180 

o  .  0098 

211 

O.OIOO 

244 

0.0133 

282 

0.0133 

T  =  29°;  L  = 

Ether. 

14.37  cc.;  p  =  Atm.  (p  —  20) 

5  Per  cent 
T  =  23.9°;L  -  7 

.  rubber  in  ether. 

.  72  cc.;  p  =  Atm.  (p  =  20) 

I 

1 

L 

, 

1 

L 

20 

0  .  OOOO 

45 

0.005 

100 

0.0137 

123 

0.017 

200 

0.0291 

195 

0.027 

300 

0.0423 

286 

0.038 

400 

0.0540 

T  =  100°;  L  = 

16.85  cc.;  p  =  Atm.  (p  =  10) 

T  =  100°;  L  =  8. 

93  cc.;  P  =  -Atm.  (p  =  10) 

P 

1 

L 

P 

1 

L 

10 

o  .  oooo 

57 

O.O2I 

IOO 

0.0357 

148 

0.051 

200 

0.0653 

221 

O.O7I 

300 

0.0876 

298 

0.088 

400 

o  .  1060 

dispersoids.  They  show  clearly,  however,  that  gelatine,  for 
example,  at  100°  is  about  10  per  cent,  less  compressible  than 
pure  water  (see  Fig.  18).  This  difference  lies  well  beyond  any 
possible  experimental  error.  A  series  of  measurements  by  H. 


120 


SPECIAL  COLLOID-CHEMISTRY 


Gilbaut1  on  the  compressibility  of  a  30  per  cent,  potassium  iodide 
solution  at  20°  is  introduced  for  comparison. 

A  more  thorough  investigation  of  the  compressibility  of  suspen- 
soids  and  emulsoids  would  be  interesting,  especially  as  dependent 
upon  concentration  and  temperature2  but  more  exact  methods 
than  those  employed  by  C.  Barus  would  have  to  be  used,  like  those 
of  Amagat  and  Gilbaut. 

The  compressibility  of  gels  cannot  be  discussed  until  their 
general  properties  have  been  taken  up. 


130 

720 

110 

100 

90 

80 

70 

60 

50 

40 

30 

20 


Water 
(at-  100°)/j0o/o 

GelaHn. 

100°y  30%KI 
(dt20°l 


OOfO* 

Atmospheres  100  200  300 

FIG.  1 8. — Compressibility  of  dispersoids.     (According  to  C.  Barus  and  H.  Gilbaut,} 

2.  Density  and  Space  Relations  in  Colloid  Systems. — That 
the  density  (weight/volume)  or  specific  volume  (volume/weight) 
of  colloid  systems  must  have  values  other  than  the  arithmetic 
mean  of  the  values  of  dispersion  medium  and  dispersed  phase 
follows  from  the  existence  of  capillary  pressure  and  from  the  com- 
pressibility relations.  The  change  in  density  may  be  calculated 

1  H.  Gilbaut,  I.e.,  422. 

2  The  relation  between  compressibility  and  temperature  in  gelatine  solutions 
has  also  been  studied  by  Barus,  but  no  values  were  obtained  which  in  his  judgment 
permitted  of  positive  deductions  regarding  any  special  behavior  of  colloid  solutions. 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS      121 

in  advance  by  the  method  of  Wilhelm  Ostwald1  if  a  positive  capil- 
lary pressure  (the  result  of  positive  or  contractile  surface  tension) 
is  present  and  size  of  the  particles,  surface  tension  and  com- 
pressibility are  known.  Such  calculations  show  that  drops  of 
water  3^  in  diameter  are  0.00005  times  denser  than  water  en 
masse.  As  previously  pointed  out  (p.  73)  this  apparently  in- 
significant value  increases  rapidly  with  further  subdivision  so 
that  drops  of  water  0.03/4  in  diameter,  possessing,  in  other  words,  a 
colloid  degree  of  dispersion  have  a  density  0.5  per  cent,  greater  than 
water  en  masse.  From  this  it  follows  that  similar  changes 
of  density  must  be  observable  whenever  coarsely  dispersed  or 
nondispersed  systems  go  over  into  colloid  ones,  as  in  the  "solu- 
tion "  of  colloids  or  the  closely  related  phenomena  of  swelling  in 
emulsoids. 

As  a  matter  of  fact  such  changes  have  been  frequently  found. 
The  experiments  of  G.  Rose2  may  be  cited  for  changes  of  density  in 
suspensoid  systems.  This  investigator  found  the  following  values 
for  gold  of  different  degrees  of  dispersion: 

Molten  and  compressed 19 . 33 

Precipitated  with  oxalic  acid 19 . 49 

Precipitated  with  ferrous  sulphate 19 . 55  to  20 . 71 

Ferrous  sulphate  precipitates  gold  as  a  very  fine  powder,  oxalic  acid 
in  the  form  of  tiny  platelets.  Similarly,  barium  sulphate  in 
lumps  showed  a  density  of  4.48;  in  precipitated  form  one  of  4.521 
and  4.535.  The  measurements  carried  out  by  J.  P.  Cholodny3 
on  colloid  selenium  and  silver  showed  similar  though  smaller 
differences.  As  has  already  been  mentioned,  the  compressibility 
of  the  substances  themselves  is  an  essential  factor  in  these  in- 
vestigations of  the  influence  of  dispersion  on  density,  and  since 
this  compressibility  is  itself  but  small  any  variations  observed  in  it 
can  also  only  be  slight. 

In  studying  the  influence  of  solid  suspended  particles  upon  the 
density  of  a  liquid,  one  encounters  complicated  relations.4  We 
can  only  say  that  the  calculated  density  taken  as  the  arithmetic 
mean  approaches  the  observed  value  more  and  more  with  increas- 

1  Wilhelm  Ostwald,  Grundriss  d.  allg.  Chem.,  4,  533  (1910  Edition). 
a  G.  Rose,  Poggendorf's  Ann.,  73,  i  (1848). 

3  J.  P.  Cholodny,  Kolloid-Zeitschrift,  2,  19,  340  (1907). 

4  See,  for  example,  the  recent  work  of  B.  Loffler,  Drude's  Ann.  d.  Phys.,  23,  3 
(1907). 


122 


SPECIAL  COLLOID-CHEMISTRY 


ing  degree  of  dispersion  of  the  solid.  This  rule,  however,  applies 
only  to  average  degrees  of  dispersion,  for  decided  deviations  are 
found  when  dealing  with  substances  in  a  state  of  colloid  sub- 
division. It  would  be  of  interest  to  have  available  a  series  of 
measurements  showing  the  dependence  of  the  density  of  a  disper- 
soid  upon  the  degree  of  dispersion. 

Measurements  of  density  changes  in  emulsoid  systems  are 
more  numerous.  By  direct  observation  in  a  dilatometer,  H. 
Quincke1  found  that  the  system,  dried  egg  albumin-water  (100 
grams  albumin  -f-  163.9  cc-  water)  showed  a  decrease  in  volume 
amounting  to  4.175  cc.  after  thirty -six  hours.  In  another  instance 
(100  grams  albumin  +  144.77  cc-  water)  a  contraction  of  5.168  cc. 
took  place.  In  the  latter  instance  the  contraction  in  volume 
amounts  to  more  than  3.5  per  cent.  The  densities  of  dried  and 
hydrated  cartilage  were  observed  to  be  as  follows:  the  calculated 
values  in  the  table  are  the  arithmetic  means. 


Dried 

Hydrated 
(observed  density) 

Hydrated 
(calculated  density) 

1.407 
1.368 

1.0892 
I  .1124 

1.0826 
I  .  1090 

Chr.  Liideking2  among  others  has  made  similar  investigations 
on  gelatine  gels,  and  H.  Rodewald3  on  starch.  The  following  table 
gives  some  of  their  findings. 

TABLE  5 


Observed  density 

Calculated  density 

Difference 

10  per  cent. 

gelatine  ...   ...... 

1  .060 

I  .0412 

O  028 

20  per  cent. 

gelatine 

I    13"? 

I    2O3 

O  O32 

50  per  cent. 

gelatine  

1  .  242 

1  .  206 

0.036 

If  the  volume  of  i  cc.  of  water  after  contraction  is  calculated 
from  these  figures  the  following  values  are  obtained: 

For  10  per  cent,  gelatine o .  96069  cc. 

For  25  per  cent,  gelatine o . 93748  cc. 

For  50  per  cent,  gelatine o .  90201  cc. 

1  H.  Quincke,  Pfliiger's  Arch., 3, 33 2  (1870);  still  earlier  observations  are  recorded 
by  W.  Schmidt,  Poggendorf's  Ann.,  114,  337  (1861). 

2  Chr.  Ludeking,  Wiedemann's  Ann.  d.  Physik.,  35,  552  (1888). 

3  H.  Rodewald,  Zeitschr.  f.  physik.  Chem.,  24,  193  (1897). 


MECHANICAL   PROPERTIES   OF    COLLOID    SYSTEMS 


123 


The  assumption  has  here  been  made  that  only  the  water  and  not  the 
gelatine  has  been  compressed,  which  is  probably  not  strictly  true. 
These  experiments  show  that  the  contraction  increases  with  the 
concentration  of  the  system.  This  is  even  more  evident  from  the 
experiments  of  H.  Rodewald  on  the  decrease  in  volume  when 
starch  absorbs  water  (see  Table  6  and  Fig.  ig).1  Here  also  the 
contraction  of  volume  increases  absolutely  with  the  concentration, 


50 

40 

30 

20 

0010 

0000 


Wafer  content- 


5  10  15  20% 

FIG.  19. — Relation  between  loss  of  volume  and  water  content  in  absorption  of  water 
by  starch.     (Calculated  from  measurements  by  H.  Rodewald.) 


only  the  concentration  has  in  this  case  not  been  referred  to  the 
disperse  phase  (starch)  but  to  the  dispersion  means  (water). 

TABLE  6. — CONTRACTION  IN  VOLUME  OF  THE  SYSTEM  STARCH-WATER 
(Referred  to  the  Volume  Contraction  of  Starch,  after  H.  Rodewald) 


HzO  content  of 
starch  in 
per  cent. 

Volume  relation 
of  starch 
(volume  /weight) 

Volume  contraction  of  starch 
between  volume  relations  of 
moist  starch 

Difference 
dried  and  of 

0.0 

0.72585 

I.I7 

0.72124 

0.00461 

2.69 

0.7I3I9 

O.OI266 

5.02 

o  .  70486 

0.02099 

7.40 

0.69881 

0.02704 

9.70 

0.69344 

0.03241 

I3-4I 

0.68727 

0.03858 

14-95 

0-68555 

o  .  04030 

19.24 

0.68151 

0.04434 

Maximum                   0.67273 

0.05312 

1  The  values  in  the  table  and  the  figure  have  been  partly  recalculated  from  Rode- 
wald's  measurements. 


124  SPECIAL  COLLOID-CHEMISTRY 

But  Fig.  19  also  shows  that  the  greatest  relative  contraction  occurs 
when  the  water  content  is  at  a  minimum,  in  other  words,  the 
first  traces  of  moisture  suffer  a  relatively  greater  contraction  than 
the  subsequent  ones.  This  is  indicated  by  the  curve  which  is 
concave  toward  the  abscissa.  It  should  be  emphasized  that 
J.  M.  van  Bemmelen  pointed  out  all  these  facts  years  ago  in  dis- 
cussing water  absorption  by  inorganic  gels.1 

In  considering  molecular  and  supermolecular  dispersoids 
attention  might  be  called  to  the  contractions  in  volume  occurring 
in  the  ordinary  process  of  solution.  The  striking  "electro- 
strictures"  which  take  place  when  electrolytes  go  into  solution 
should  be  especially  mentioned.2  Whether  increases  in  "volume 
dependent  purely  upon  mechanical  effects  are  of  general  occurrence 
in  dispersed  systems  cannot  be  definitely  stated  at  the  present 
time,  for  in  dispersive  processes  which  are  associated  with  increase 
of  volume,  as  in  the  solution  of  ammonium  salts  in  water,  both 
chemical  and  electrochemical  changes  occur  in  the  dispersed  phase. 

3.  The  Concentration  Function  of  Density  in  Colloid  Systems. 
—The  few  observations  thus  far  available  on  the  relation  of 
density  to  concentration  in  colloids  show  an  interesting  difference 
between  suspensoids  and  emulsoids.  The  figures  of  S.  E.  Linder, 
H.  Picton8  and  G.  Quincke4  which  are  given  in  Table  7  and  Fig. 
20  show  that  in  suspensoid  systems  the  change  in  density  is 
strictly  proportional  to  the  concentration;  in  other  words,  the 
function,  density-concentration,  is  linear  and  the  corresponding 
curve  a  straight  line.  Fig.  20  shows  how  accurately  this  applies  to 
the  sol  of  arsenious  trisulphide.  In  contrast  herewith,  the  density- 
concentration  curve  of  gelatine  shows  a  decided  curvature,  concave 
toward  the  ordinate.  This  means  that  the  increase  in  density  is 
relatively  less  in  lower  than  in  higher  concentrations.  This 
makes  a  new  and  important  distinction  between  suspensoids  and 
emulsoids  which  might  well  be  studied  experimentally  in  greater 
detail.  The  numerical  values  betray  the  curvature  even  more 
plainly  than  their  graphic  representation,  for  if  the  increases  in 
density  with  unit  increases  in  Concentration  are  compared,  it  is 
found  that  the  density  increases  uniformly  with  increasing  con- 

1  J.  M.  van  Bemmelen,  Gesam.  Abhandl.,  275,  458,  Dresden,  1910. 

2  See  the  textbooks  of  physical  chemistry  for  further  details. 

3  S.  E.  Linder  and  H.  Picton,  Journ.  Chem.  Soc.,  67,  71  (1895). 

4  G.' Quincke,  Drude's  Ann.  d.  Physik.,  9,  800  (1907);  10,  486,  809  (1903). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


12$ 


FIG.  20. — Relation  of  density  to  concentration  in  colloids. 
TABLE  7. — CONCENTRATION-FUNCTION  OF  DENSITY  IN  COLLOID  SOLUTIONS 


As2S3  sol 
andH 

(S.  E.  Linder 
.  Picton) 

(G° 

-gelatine 
Quincke) 

/3-gelatine 
(G.  Quincke) 

NaCl 
(Karsten*) 

Concen- 

Concen- 

Den- 

Concen- 

Den- 

Concen- 

Den- 

tration, 

Density 

tration 

, 

sity 

A 

tration, 

sity           A 

tration, 

sity 

A 

per  cent. 

per  cent. 

per  cent. 

per  cent 

0.0172 

1.000137 

5 

i 

.014 

5 

1  .014 

I 

I  .  6064 

28 

3° 

355 

o  .  0344 

1.000267 

10 

i 

.028 

10 

1.030 

5 

1-0355 

0.0688 

1.000535 

20 

i 

•054 

26 

20 

1  .060 

30 

10 

I  .0726 

37i 

34 

34 

379 

0.1375 

1.001050 

30 

i 

.088 

1 

30 

1.094 

15 

1.1105 

34 

38 

392 

0.2750 

I.O02IIO 

40 

i 

.  122 

40 

1.132 

20 

I  .  1407 

'    44 

40 

407 

0.5500 

I.OO42OO 

50 

I 

.166 

50 

1.172 

25 

I  .  1904 

I  .  1000 

1.008435 

2  .  2OOO 

I.  Ol6o8o 

Silicic  acid 
(G.  Quincke) 

Tannin  (aqueous) 
(G.  Quincke) 

4  .  4000 

I.0338IO 



- 

I 

i 

.005 

2.5 

I  .OIOI 

29 

200 

3 

i 

.017 

5-o 

i  .0200 

5 

I 

.029 

IO.O 

1.0409 

209 

30 

10 

I 

•°59 

1  From  Landolt-Bornstein,  Physik.-Chemisch.  Tabellen,  3  Aufl.,  322. 


126  SPECIAL  COLLOID-CHEMISTRY 

centration  (see  under  A  in  the  third  column  of  Table  7).  In 
future  determinations  of  the  density-concentration  functions  of 
colloids  this  sensitive  mathematical  proof  should  be  given  due 
preference. 

It  should  also  be  emphasized  that  the  density-concentration 
curve  of  molecular  dispersoids,  especially  that  of  electrolytes,  is 
concave  toward  the  density  axis  (see  NaCl  in  Table  7  and  Fig. 
20).  This  is  already  apparent  from  the  fact  that  the  contraction 
of  volume  on  dilution  of  salt  solutions,  for  example,  is  most 
pronounced  in  concentrated  solutions  and  becomes  less  with 
increasing  dilution.  When  a  concentrated  salt  solution  is  di- 
luted one-half  and  this  dilution  is  again  diluted  one-half,  etc., 
the  observed  contraction  becomes  progressively  less.  This 
corresponds  to  the  two  concave  curves  of  Fig.  20.  Here  again 
there  are  evident  interesting  parallels  between  the  properties  of 
emulsoids  and  of  molecularly  dispersed  solutions  of  salts,  etc. 
Attention  has  been  called  to  such  similarities  before  and  the 
occasion  will  arise  again  later. 

The  older  measurements  of  W.  Schmidt  (I.e.)  and  Ch.  Liide- 
king  (I.e.)  were  doubtlessly  carried  out  with  impure  material's 
so  that  they  cannot  be  considered  here.  Thus  the  density  of  a 
20  per  cent,  solution  of  gelatine,  according  to  Liideking,  is  1.130 
while  according  to  Quincke  it  is  only  1.054.  This  indicates, 
even  if  we  make  allowance  for  possible  temperature  differences, 
that  Liideking  used  a  gelatine  much  richer  in  salts  than  did 
Quincke.  A  measurement  of  the  concentration-function  when  the 
salt  content  is  known,  would  perhaps  be  interesting  for  the  theory 
of  the  internal  changes  in  state  of  gelatine,  in  other  words,  in  a 
study  of  the  influence  which  gelatine  and  dissolved  salt  have  upon 
each  other. 

4.  Thermal  Coefficient  of  Expansion  in  Colloids. — Observa- 
tions analogous  to  the  above  may  be  made  on  the  thermal  (cubic) 
coefficients  of  expansion  in  colloids.  Here  also  we  would  antici- 
pate the  behavior  to  be  analogous,  roughly,  to  that  of  molecular 
dispersoids  but  the  absolute  changes  in  the  constants  of  the  pure 
dispersion  medium  in  the  case  of  colloids  would  not  be  expected  to 
be  as  great  as  in  the  case  of  molecular  dispersoids.  We  would 
also  expect  the  emulsoids  to  bear  a  closer  resemblance  to  molecu- 
larly and  ionically-dispersed  systems  than  the  suspensoids. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  127 

In  this  connection  it  must  be  specially  emphasized  that  the 
change  in  volume  of  pure  water  with  increasing  temperature 
above  40° C.  is  represented  by  a  curve  concave  toward  the  tem- 
perature axis.  In  the  case  of  a  salt  solution  this  curve  is  flatter,  in 
fact  it  tends  in  high  concentrations  to  approximate  a  straight  line.1 
The  form  of  the  curve  indicates  that  molecularly  dispersed 
aqueous  systemsexp  and  more  at  lower  temperatures  than  water, 
and  less  than  this  at  higher  temperatures.  There  exists  a  region 
therefore  in  which  the  expansion  coefficient  of  solution  and  of 
dispersion  means  are  the  same.  The  curves  for  the  solution  and 
for  the  pure  dispersion  medium  cross  each  other  as  shown  dia- 
grammatically  in  Fig.  21.  For  NaCl  this  temperature  is  about 
55°.  According  to  M.  P.  de  Heen2  the  point  of  intersection  de- 
pends upon  the  nature  of  the  salt  only,  and  not  upon  its  concen- 
tration. It  would  be  interesting  to  determine  how  colloid  solu- 
tions behave  in  this  respect. 

From  the  meager  material  available  the  following  figures  of 
Rodewald  are  here  reproduced,  on  the  average  thermal  coefficient 
of  expansion  of  starch  with  varying  water  content.  The  coeffi- 
cient of  expansion  a  is  the  ratio  of  volume  increase  to  original 
volume.  The  temperatures  employed  range  between  o°  and  20° C. 

TABLE  8. — THERMAL  EXPANSION  COEFFICIENT  OF  THE  SYSTEM  STARCH-WATER 

(After  Rodewald) 

Water  content  of  Thermal  coefficient 

starch,  per  cent.  of  expansion 


o.oo  0.000104 

10.23  0.000167 

J  0.000236 
0.000240 

41.05  ("saturated"  with  H2O)  0.000383 


10.19  ("air  dry") 


The  calculated  values  and  their  graphic  portrayal  show  that 
the  thermal  coefficient  of  expansion  increases  rectilinearly  with  the 
water  content.  It  must  be  borne  in  mind  that  in  starch  we  are  not 
dealing  with  a  typical  colloid,  but  with  a  coarsely  disperse  system 
which  assumes  a  typical  colloid  character  only  after  becoming  very 
rich  in  water. 

Though  we  possess  but  few  measurements  on  pure  colloid 
solutions  we  have  many  experimental  data  on  the  thermal  expansion 

1  See  Wilh.  Ostwald,  Lehrb.  d.  allg.  Chem.,  2  Aufl.,  791,  Leipzig,  1903. 

2  See  Wilh.  Ostwald,  I.e. 


128 


SPECIAL  COLLOID-CHEMISTRY 


of  a  special  type  of  colloid  system,  namely,  the  gels  of  rubber  and 
gelatine.  But  these  exhibit  complicated  and  special  properties 
which  can  be  discussed  to  advantage  only  after  the  general  proper- 
ties of  gels  have  been  taken  up. 


Water 


Sail- 
Sol  u  Ho  n 


Temperature * 

FIG.  21. — Diagram  illustrating  relation  of  changes  in  volume  to  changes  in 
temperature  in  water  and  in  salt  solution. 


§22.  Vapor  Tension,  Boiling  Point  and  Freezing  Point  of  Colloid 

Solutions 

i.  General  Remarks. — Among  the  most  important  physical 
changes  which  a  liquid  (or  solid)  dispersion  medium  suffers  in 
taking  up  a  molecularly  or  ionically  dispersed  phase,  are  the  lowering 
of  its  vapor  tension  and  its  freezing  point;  and  the  elevation  of 
its  boiling  point.  The  great  importance  of  these  phenomena  resides 
in  the  simple  quantitative  relations  which  exist,  according  to  the 
investigations  of  A.  Wiillner,  F.  Raoult,  and  others,  between  the 
amount  of  these  changes  and  the  concentration  of  the  molecularly 
dispersed  phase.  These  relations,  together  with  the  laws  of  osmosis, 
have  been  combined  by  J.  H.  van't  Hoff  in  his  classic  theory  of 
true  solutions.  It  might  now  be  asked  if  colloids  show  a 
corresponding  behavior.  Investigations  of  this  character  were 
made  long  ago.  The  result  may  be  expressed  thus:  Pure  col- 
loids (that  is  such  as  are  as  free  as  possible  from  molecular  dis- 
persoids)  affect  only  very  slightly  the  vapor  pressure,  freezing  point, 
and  boiling  point  of  the  dispersing  medium.  It  even  seems  as  if 
some  colloids  which  can  be  obtained  in  a  high  state  of  purity  do 
not  influence  these  properties  at  all. 

To  the  above  statement  must  be  added  another  more  general 


MECHANICAL  PROPERTIES  OP  COLLOID  SYSTEMS      I2Q 

and  perhaps  even  more  important  one :  The  amount  of  change  in 
these  properties,  more  particularly  in  the  freezing  point,  in  the  vapor 
pressure  and  in  the  boiling  point,  depends  upon  the  degree  of  dis- 
persion of  the  system,  and  increases  with  every  increase  in  the  degree 
of  dispersion.  A  ready  method  by  which  a  steady  change  in  the 
degree  of  dispersion  may  be  produced  is  to  vary  the  relative 
amounts  of  dispersion  medium  and  disperse  phase.  In  support  of 
this  general  statement  may  be  cited  the  anomalous  behavior  of 
molecular  dispersoids  in  concentrated  solution  commonly  accounted 
for  through  association,  polymerization,  etc. ;  further,  as  previously 
mentioned,1  " concentration-variable  dispersoids"  are  by  no  means 
rare.  To  this  important  class  of  systems  belong  most  of  the  emul- 
soids,  as  the  chapters  on  " internal  changes  in  state,"  "gelation," 
and  " swelling"  will  clearly  show.  Thus  F.  Krafft2  and  A.  Smits3 
found  that  dilute  soap  solutions  showed  quantitatively  determin- 
able  elevations  of  the  boiling  point  and  depressions  of  the  vapor 
pressure  which  became  progressively  less  with  increase  in  the  con- 
centration of  the  solution  until  they  finally  became  zero.  The 
measurements  of  A.  Smits,  upon  which  is  based  the  law  stated  in 
the  first  part  of  this  paragraph  are  reproduced  in  the  following 
table: 

TABLE  9. — AQUEOUS  SOLUTIONS  OF  SODIUM  PALMITATE 


Molar  concentration      Elevaitni°npofntb°il 


Molar  concentra- 
tion 


Reduction  of  vapor  pressure 


0.0282 

0.024 

0.50 

1.3  mm.  Hg. 

0.1128 

0.045 

o-7S 

0-5 

0.2941 

0.050 

i  .00 

o.o 

0.5721 

0.060 

It  is  especially  interesting  to  note  that  though  the  elevation 
of  the  boiling  point  increases  with  increasing  concentration,  even 
though  but  little,  the  lowering  of  the  vapor  pressure  decreases 
absolutely  at  higher  concentrations.  It  is  therefore  evident  that 
the  two  kinds  of  changes  in  no  sense  parallel  each  other,  much  less 
that  a  mathematical  proportionality  exists  between  them. 

2.  Measurements  of  Vapor  Pressure  of  Colloid  Solutions. — 
Other  measurements  of  vapor  pressure,  besides  those  carried  out 

1  See  p.  35. 

2  F.  Krafft,  Ber.  d.  Dtsch.  Chem.  Ges.,  29,  1328  (1896). 

3  A.  Smits.  Z.  f.  physik.  Chem.,  45,  608  (1903). 

9 


130  SPECIAL  COLLOID-CHEMISTRY 

by  A.  Smits  have  been  made  by  F.  Guthrie,1  Ch.  Liideking,2  G. 
Tammann,3  G.  Bruni  and  N.  Pappada,4  and  D.  Konowalow. 
F.  Guthrie  thought  he  could  observe  an  evident  depression 
of  the  vapor  tension  in  gelatine  solutions.  But  Llideking, 
who  carefully  repeated  these  experiments,  obtained  only  nega- 
tive results.  He  justly,  no  doubt,  attributed  the  results  of  the 
English  investigator  to  impure  material.  Tammann  found  that 
even  large  additions  of  gelatine,  tragacanth,  gum  arabic,  etc., 
changed  the  vapor  pressure  of  water  but  slightly,  and  that  con- 
centration did  not  appear  to  influence  the  result.  Considering 
the  behavior  of  soap  solutions,  however,  it  does  not  appear 
impossible,  that  with  great  dilution  the  above  substances  would 
show  marked  changes  in  the  properties  under  discussion,  on 
account  of  the  greater  degree  of  dispersion.  Negative  results, 
similar  to  those  of  Tammann  were  obtained  by  G.  Bruni  and 
N.  Pappada. 

Konowalow  carried  out  investigations  with  mixed  liquids  in  the 
critical  area.  He  found  that  these  systems  which  on  the  basis 
of  the  discussion  on  p.  47,  we  must  regard  as  highly  disperse 
emulsoids,  behaved  in  a  manner  entirely  analogous  to  colloid  solu- 
tions. In  these  critical  areas  the  reduction  of  the  vapor  tension 
is  largely  independent  of  the  concentration. 

The  works  of  J.  M.  van  Bemmelen5  furnish  further  measure- 
ments of  the  vapor  pressure  of  colloid  systems.  These  meas- 
urements do  not  pertain  to  sols,  however,  but  to  precipitated 
colloids,  in  other  words  to  gels.  Such  systems  cannot,  however, 
be  compared  directly  with  molecularly  disperse  systems,  for  they 
are  structures  of  an  extremely  variable  character;  in  other  words, 
they  are  subject  to  great  changes  in  state.  A  discussion  of  the 
vapor  tension  of  these  gels  will  therefore  more  properly  appear 
later. 

3.  Elevation  of  Boiling  Point  of  Colloid  Solutions. — Boiling 
point  determinations  have  been  made  on  colloid  solutions  by  F. 
Guthrie,6  Ch.  Llideking,7  A.  Sabanejew,8  S.  E.  Linder  and  H. 

1  F.  Guthrie,  Philosoph.  Mag.  (5),  2,  219  (1876). 

*  Ch.  Liideking,  Wiedemann's  Ann.  d.  Physik.,  35,  552  (1888). 

3  G.  Tammann,  Mem.  de  1'Acad.  de  St.  Petersburg  (7),  35. 

4  G.  Bruni  and  N.  Pappada,  Rend.  tec.  Lincei  (5),  9,  I,  354  (1901);  Gaz.  chim. 
ital.,  31  (1901). 

6  J.  M.  van  Bemmelen,  Die  Absorption.  Ges.  Abhandl.,  Dresden,  1910. 

6  F.  Guthrie,  Philosoph.  Mag.  (5),  2,  211  (1876). 

7  Ch.  Ludeking,  Wiedemann's  Ann.  d.  Physik.,  35,  552  (1888). 

8  A.  Sabanejew,  J.  d.  Russ.  physik.-chem.  Ges.,  22,  102  (1890). 


MECHANICAL  PROPERTIES   OF   COLLOID    SYSTEMS  131 

Picton,1  A.  Lottermoser,2  F.  Krafft  (I.e.),  A.  Smits  (I.e.),  and 
many  others.  All  the  values  obtained  were  exceedingly  small, 
and  became  even  smaller  as  the  purity  of  the  employed  colloid 
solution  increased.  This  applies  both  to  emulsoids  (gelatine, 
silicic  acid,  dextrine,  rubber,  starch,  etc.),  and  to  suspensoids 
(arsenious  trisulphide  hydrosol,  mercuric  sulphide  hydrosol, 
stannic  acid  hydrosol,  etc.). 

4.  Depression  of  Freezing  Point  of  Colloid  Solutions. — 
Because  of  the  relative  ease  with  which  the  experimental  work 
of  determining  the  freezing-point  depression  is  carried  out,  such 
determinations  have  been  made  very  frequently.  Deserving 
of  particular  mention  are  those  of  H.  F.  Brown  and  G.  H. 
Morris,3  J.  H.  Gladstone  and  W.  Hibbert,4  A.  Sabanejew,5 
E.  Paterno,6  N.  Ljubavin,7  A.  Sabanejew  and  N.  Alexandro,8 
S.  E.  Linder  and  H.  Picton,9  C.  E.  Linebarger,10  G.  Tam- 
mann,11  W.  Meyer,12  St.  Bugarsky  and  L.  Liebermann,13  H. 
Friedenthal,14  F.  Krafft,15  A.  Lottermoser,16  N.  Pappada,17 
T.  Koerner  and  P.  Dullberg,18  Z.  Gatin-Gruszewska,19  W.  R. 
Whitney  and  J.  Blake,20  G.  Malfitano  and  Michel,21  F.  Bottazzi 
and  G.  D'Enrico,22  T.  B.  Robertson  and  Th.  C.  Burnett,23  J. 
Duclaux24  and  G.  Moruzzi.25  The  results  were  entirely  analogous 

1  S.  E.  Linder  und  H.  Picton,  Journ.  Chem.  Soc.,  61,  114  (1892). 

2  A.  Lottermoser,  Anorganische  Kolloide,  74,  Stuttgart,  1901. 

*H.  T.  Brown  und  G.  H.  Morris,  Journ.  Chem.  Soc.,  53,  610  (1888). 
4  J.  H.  Gladstone  und  W.  Hibbert,  Philos.  Mag.  (5),  28,  38  (1889). 
8  A.  Sabanejew,  Journ.  d.  Russ.  physik.-chem.  Ges.,  21,  515  (1889)  und  22,  102 
(1890);  Ber.  d.  Dtsch.  chem.  Ges.,  23,  87  (1890);  24,  558  (1891). 

6  E.  Paterno,  Z.  f.  physik.  Chem.,  4,  457  (1889). 

7  N.  Ljubavin,  Journ.  d.  Russ.  physik.-chem.  Ges.,  21,  397  (1889). 

8  A.  Sabanejew  und  N.  Alexandrow,  ibid.,  23,  7  (1891). 

9  S.  E.  Linder  und  H.  Picton,  Journ.  Chem.  Soc.,  61,  114  (1892). 

10  C.  E.  Linebarger,  Silliman's  Am.  Journ.  (3),  43,  416  (1892). 

11  G.  Tammann,  Zeitschr.  f.  physik.  Chem.,  20,  180  (1896). 

12  W.  Meyer,  Zur  Kenntnis  einiger  anorgan.  Kolloidsubstanzen.  Diss.  Halber- 
stadt  (1897). 

13  St.  Bugarsky  und  L.  Liebermann,  Pfliiger's  Arch.  f.  Physiol.,  72,  51  (1898); 
u.  Tangl,  ibid.,  72,  531  (1898). 

14  H.  Friedenthal,  Physiol.  Zentralbl,  12,  849  (1899). 

15  F.  Krafft,  Ber.  d.  Dtsch.  chem.  Ges.,  32,  1614  (1899). 

16  A.  Lottermoser,  Anorganische  Kolloide.,  74,  Stuttgart,  1901. 

17  N.  Pappada,  Gazz.Chim.Ital.,32  (II),  22  (1902);  auch  G.  Bruni  u.  N.  Pappada 
(l.c.,igoi). 

18  T.  Korner  und  P.  Dullberg,  Deutsch.  Gerberztg.,  47  (1904). 

19  Z.  Gatin-Gruszewska,  Pfluger's  Arch.  f.  Physiol,  102,  569  (1904). 

20  W.  R.  Whitney  und  J.  Blake,  Journ.  Amer.  Chem.  Soc.,  26,  1339  (1904). 

21  G.  Malfitano  und  Michel,  Compt.  rend.,  143,  1141  (1907). 

22  F.  Bottazzi  und  G.  D'Enrico,  Pfliiger's  Arch.,  1/15,  359  (1906). 

83  T.  B.  Robertson  u.  Th.  C.  Burnett,  Journ.  Biol.  Chem.,  6,  105  (1909). 

24  J.  Duclaux,  Compt.  rend.,  148,  714  (1909). 

25  G.  Moruzzi,  Bioch.  Zeitschr.,  22,  232  (1809). 


132  SPECIAL  COLLOID-CHEMISTRY 

to  those  obtained  in  determinations  of  the  lowering  of  the  boiling 
point.  One  fact  stands  out  clearly,  however.  Owing  to  the 
greater  number  of  substances  that  have  been  investigated, 
colloid  solutions  have  been  found  which  show  an  undoubted  de- 
pression of  the  freezing  point.  This  is  easily  understood,  if  we 
bear  in  mind  the  variation  which  colloids  show  in  their  degree  of 
dispersion . 

An  interesting  study  has  been  made  by  H.  Friedenthal  (I.e.) 
of  the  molecular  weight  of  the  so-called  soluble  starch.  This 
starch  may  be  prepared  in  various  ways,  among  others  by  treat- 
ing common  starch  with  ozone.  The  product  so  obtained  appears 
in  every  way  to  be  "depolymerized,"  that  is,  to  be  more  highly  dis- 
persed than  ordinary  starch.  This  is  betrayed  by  the  slight,  but 
nevertheless  definite  depression'  of  the  freezing  point  exhibited  by 
the  soluble  starch,  in  contrast  to  ordinary  starch,  as  the  following 
table  shows. 

TABLE  10. — DEPRESSION  OF  THE  FREEZING  POINT  OF  SOLUBLE  STARCH 
(According  to  H.  Friedenthal) 

Concentration,  per  cent.  Depression  of  the  freezing  point 


2-5  0.005 

5.0  o.Ol 

IO.O  O.O2 


There  even  exists  a  proportionality  between  concentration  and 
freezing-point  depression.  The  calculated  molecular  weight  of 
soluble  starch  is  about  9450,  while,  according  to  H.  T.  Brown 
and  G.  H.  Morris  (I.e.)  that  of  normal  starch  is  at  least  32,400. 
These  freezing-point  measurements  have  also  shown  that  the  ob- 
served depressions  of  the  freezing  point  have  decreased  as  the 
solutions  under  investigation  have  been  purified  through  dialysis, 
etc.  Gatin-Gruszewska  (I.e.)  found  solutions  of  very  pure  glyco- 
gen  to  show  no  depression  of  the  freezing  point  whatsoever. 

The  relations  existing  between  changes  of  vapor  pressure, 
boiling  point  and  freezing  point  of  colloids,  and  their  molecular 
weights  will  be  discussed  later. 

§23.  Mass-relations  in  Colloids 

i.  Concentration  of  Colloid  Systems. — The  concentration  of  a 
disperse  system  is  expressed  by  the  quantitative  relation  of  the 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS  133 

disperse  phase  to  the  dispersing  medium.  This  relation  is  more 
complex  in  dispersoids,  particularly  in  colloids,  than  in  molecular 
dispersoids,  and  for  the  following  reasons. 

In  molecular  dispersoids  such  as  the  "true"  solutions  of  any  non- 
electrolyte,  we  may  assume  that  in  a  given  dispersion  means  the 
disperse  phase  will  always  have  the  same  degree  of  dispersion ;  in 
other  words,  the  disperse  phase  will  always  assume  a  state  of 
maximum  or  molecular  subdivision.  This  assumption  is  based 
upon  the  validity  of  the  gas  laws  and  Avogadro's  hypothesis. 
The  assumption  no  longer  holds,  of  course,  when  we  deal  with 
electrolytes,  or  with  molecular  dispersoids  of  such  concen- 
tration that  a  change  in  the  degree  of  dispersion,  such  as 
" polymerization,"  etc.,  takes  place;  or  when  van't  Hoff's  laws  are 
no  longer  strictly  obeyed.  And  yet  all  these  factors,  which  tend 
to  make  ambiguous  the  term  "concentration"  when  applied  to  the 
proportionality  existing  between  amount  of  molecular  dispersoid 
and  dispersing  medium,  are  but  slight  in  their  effects  when  com- 
pared with  the  influence  of  the  degree  of  dispersion  upon  the 
properties  of  the  coarser  dispersoids,  particularly  the  colloids. 
For  as  has  been  repeatedly  emphasized  in  the  first  chapters  of  this 
book,  it  is  the  specific  surface  which  primarily  gives  to  disperse 
systems  their  characterizing  properties.  The  same  relative 
proportion  between  the  mass  of  dispersion  medium  and  disperse 
phase  may  therefore  exist  in  two  disperse  systems;  in  other  words, 
they  may  have  the  same  concentration,  and  yet  in  other  respects 
show  an  entirely  different  behavior  due  to  differences  in  degree  of 
dispersion.  A  given  amount  of  gold,  for  example,  may,  at  one  time, 
be  distributed  in  a  given  amount  of  dispersing  medium  in  the 
form  of  a  coarse  suspension,  at  another  in  the  form  of  a  gold- 
sol,  in  a  third  as  an  ionic  dispersoid.1 

To  the  above  must  be  added,  that  many  dispersoids,  particu- 
larly colloids,  are  concentration- variable  systems;  that  in  other 
words,  they  often  change  their  state  when  merely  diluted  with 
a  given  dispersion  medium.  This  question  will  be  handled  in 
detail  later.  Variation  with  concentration  appears  to  be  the 
rule  with  emulsoids,  as  already  indicated  on  p.  47.  Such  varia- 
tions are  not  impossible,  however,  in  suspensoids.  Thus  J.  Reis- 

*See  The  Svedberg,  Koll.-Zeitschr.,  4,  168  (1909). 


134  SPECIAL  COLLOID-CHEMISTRY 

sig1  found  in  his  ultramicroscopic  investigations,  that  the  number 
of  particles  was  not  proportional  to  the  content  of  colloid  material, 
but  that  relatively  more  particles  became  visible  in  diluted  solu- 
tions. While  principles  of  optics  have  been  marshalled  to  explain 
this  behavior2  it  nevertheless  appears  possible  that  a  division  of 
the  disperse  phase  into  particles  more  highly  disperse  takes  place, 
just  as  conversely  the  union  of  smaller  particles  to  form  larger 
aggregates  on  increase  of  concentration  has  been  directly  observed 
(seep.  88). 

It  follows,  therefore,  that  the  definition  of  concentration  in  dis- 
persoids  which  do  not  possess  a  maximum  (molecular  or  ionic) 
degree  of  dispersion,  will  require  an  additional  clause  expressing  the 
degree  of  dispersion  of  the  disperse  phase.  Strictly  speaking,  all 
statements  of  the  concentration  of  colloid  systems  ought  to  be  given 
in  some  such  manner  as  follows:  Gold-hydrosol  of  x  per  cent,  gold 
content  and  x.ic/  dispersion  (or  specific  surface). 

2.  Experimental  Work  on  Saturation  in  Colloid  Solutions. — 
In  dealing  with  molecular  dispersoids  we  are  in  the  habit  of 
expecting  to  encounter  a  maximal  relation  between  dispersion 
medium  and  disperse  phase,  in  other  words,  we  expect  to 
encounter  a  saturation  concentration.  Does  such  a  saturation 
concentration  exist  in  more  coarsely  disperse  systems  such  as 
colloid  systems. 

Every  investigator,  who  has  concerned  himself  with  the  prepa- 
ration of  colloid  solutions  is  familiar  with  the  fact  that  pure, 
two-phase  systems  are  frequently  stable  only  within  narrow  limits 
of  concentration.  It  is  well  known  that  suspensoids  in  particular 
can  be  prepared  only  in  low  concentrations,  unless  a  third  "  pro- 
tecting" phase  is  present.  Although  precise  measurements  are 
lacking,  we  find  that  most  of  the  hydrosols  described  in  the  litera- 
ture contain  less  than  J£  per  cent,  of  the  metal  by  weight. 

The  maximum  silver  content  of  a  silver  sol,  according  to  A.  J. 
Prange3  is  0.475  per  cent,  by  weight;  that  of  gold  sols  fluctuates, 
according  to  L.  Vanino4  between  0.0002  per  cent,  and  0.06  per  cent. 

1  J.  Reissig,  Ultramikroskop.  Beobacht.  Diss.,  Erlangen,  1908.     Review  in  KolL- 
Zeitschr.,  5,  265  (1909). 

2  See  the  paper  of  Reissig  and  the  paragraphs  in  this  volume  on  ultramicroscopy. 

3  A.  J.  Prange,  Rec.  Trav.  chim.  Pays-Bas,  9,  121  (1890). 

4L.  Vanino  and  co-workers,  Koll.-Zeitschr.,  2,  272  (1907);  2,  52  (1907). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  135 

by  weight;  according  to  J.  Donau1  it  fluctuates  between  0.0002  per 
cent,  and  0.05  per  cent.  R.  Zsigmondy2  thinks  that  0.12  per  cent, 
is  the  maximum  concentration,  and  G.  Bredig,3  who  prepared  the 
gold  sol  by  the  electrical  method  regards  0.014  per  cent,  to  be 
about  the  maximum.  It  appears,  therefore,  that  the  "  saturation- 
concentration"  for  colloid  gold  lies  between  o.i  and  0.2  per  cent. 
Other  metal  hydrosols  gave  similarly  low  ranges  of  concentration. 
M.  Traube-Mengarini  and  A.  Scala4  found  that  the  spontaneous 
solution  of  lead  in  distilled  water  yielded  a  maximum  content  of 
colloid  lead  equal  to  0.0069  Per  cent,  after  42  hours;  after  72  hours, 
0.0078  per  cent.,  after  96  hours,  0.0092  per  cent.,  and  after  3 
months,  0.0089  Per  cent.  Ultimately,  therefore,  a  constant  lead 
content  was  obtained.  But  these  values  hold  only  when  we  deal 
with  pure  metal  sols;  when  "protecting  colloids"  are  used  the 
concentrations  may  rise  much  higher. 

The  above  statements  agree  in  bringing  out  the  fact  that  the 
metal  content  of  metal  hydrosols  is  always  low.  This  statement 
could  be  further  supported  by  additional  references  to  the  litera- 
ture. In  contrast  herewith  stand  the  results  of  P.  J.  Cholodny,5 
W.  R.  Whitney  and  J.  Blake,6  who  obtained  stable  gold  and  silver 
sols  of  incomparably  higher  concentration.  Thus  P.  J.  Cholodny 
claims  to  have  obtained  pure  silver  sols  containing  more  than  30 
per  cent,  silver.  This  high  concentration  is  so  unusual  that 
without  making  confirmatory  investigations,  one  would  be  inclined 
to  assume  that  it  is  due  to  an  admixture  of  impurities,  such 
as  emulsoid  silver,  or  iron  hydroxide ;  or  that  one  is  dealing  with  a 
coarse  suspension.  Still  more  remarkable  is  the  finding  of  Whitney 
and  Blake  who  worked  with  red  colloid  gold  prepared  by  the 
reduction  of  an  ethereal  gold  chloride  solution  with  acetylene 
gas.7  By  means  of  an  electric  current  they  concentrated  this  to 
a  thick  red  "mud."  According  to  the  authors  this  "mud"  when 
stirred  up  with  distilled  water  again  gave  colloid  solutions 
similar  to  the  original  but  of  high  concentration.  They  managed 

1  J.  Donau,  6st.  Monatsh.  f.  Chem.,  26,  525  (1905). 
2R.  Zsigmondy,  Liebigs  Arm.,  301,  33  (1898). 
3G.  Bredig,  Zeitschr.  f .  angew.  Chem.,  951  (1898). 
4M.  Traube-Mengarini  and  A.  Scala,  KoU. -Zeitschr.,  6,  249  (1910). 
6  P.  J.  Cholodny,  Journ.  russ.  phys.-chem.  Ges.,  35,  585  (1903);  abstracted  in 
KoU. -Zeitschr.,  2,  340  (1908). 

6  W.  R.  Whitney  and  J.  Blake,  Journ.  Amer.  Chem.  Soc.,  26,  1339  (1904). 
7J.  Blake,  Silliman'  Am.  Journ.  Sci.,  16,  38  (1903). 


136  SPECIAL  COLLOID-CHEMISTRY 

in  this  way  to  obtain  systems  having  a  metal  content  of  several 
per  cent.  But  here  again  the  resulting  solutions  had,  no  doubt, 
but  a  low  dispersion  value,  although  the  red  color  would  in  itself 
not  warrant  such  a  conclusion  (see  for  example  Table  2,  p.  32).  A 
repetition  of  these  investigations  with  the  aid  of  the  ultramicro- 
scope,  which  was  not  available  when  the  above-named  authors 
did  their  work,  would  certainly  prove  interesting. 

Somewhat  higher  concentrations  of  colloids  of  a  suspensoid 
character  but  not  of  metals  have  been  observed,  although,  as  a 
rule,  such  concentrated  solutions  are  rather  unstable,  their  colloid 
phase  tending  to  precipitate  in  coarsely  disperse  form.  S.  E. 
Linder  and  H.  Picton  (I.e.)  obtained  a  sol  of  arsenious  sulphide 
containing  4.4  per  cent,  by  weight  of  the  disperse  phase.  Colloid 
sulphur  with  water  as  the  dispersing  medium  may  be  prepared, 
according  to  M.  Raffo,1  containing  up  to  4.58  per  cent,  of  sulphur, 

In  contrast  to  the  suspensoids,  the  typical  emulsoids  exhibit 
no  upper  limit  of  concentration  corresponding  to  a  saturation 
concentration.  Gelatine,  silicic  acid,  or  egg-albumin  will,  with 
proper  regulation  of  the  temperature,  take  up  progressively  greater 
or  smaller  amounts  of  water,  without  separating  out  in  coarsely 
disperse  form.  The  explanation  of  this  behavior,  which  is  so 
different  from  that  of  the  suspensoids,  is  that  the  typical  emulsoids 
represent  complex  concentration-variable  systems,  or,  expressed 
more  simply,  mixtures  of  mutually  soluble  components. 

Here  again  those  systems  that  form  transitions  between 
suspensoids  and  emulsoids  are  particularly  interesting.  Thus, 
the  hydroxides,  especially  the  hydroxides  of  iron,  seem  to  behave 
like  suspensoids  in  dilute  solution,  and  like  emulsoids  in  concen- 
trated solution  (see  §9).  Because  of  this,  iron  hydroxide  sol 
of  liquid  character  may  be  obtained  in  concentrations  charac- 
teristic of  suspensoids  (up  to  4.8  per  cent,  according  to  G. 
Geffcken2) ;  while  on  the  other  hand,  a  jelly-like  emulsoid  modifi- 
cation of  iron  hydroxide  is  known,  which  contains  but  a  few 
per  cent,  of  water.3 

3.  Theoretical  Considerations  Bearing  on  the  Saturation  of 
Colloids. — In  the  older  literature  one  frequently  encounters  the 
statement  that  colloid  solutions  differ  from  the  molecularly 

1M.  Raffo,  Koll.-Zeitschr.,  2,  358  (1908). 

2  G.  Geffcken,  Zeitschr.  f.  physik.  Chem.,  49,  299  (1904). 

3  See,  for  example,  J.  M.  van  Bemmelen,  Die  Absorption,  Dresden,  1910. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  137 

disperse  in  lacking  a  saturation  concentration.  The  above  facts 
show  that  this  is  not  borne  out  by  experience,  at  least  not  as 
far  as  suspensoids  are  concerned.  In  recent  years  a  number  of 
investigators  besides  myself,  namely,  J.  Duclaux,1  W.  M.  Bayliss,2 
M.  Traube-Mengarini,  and  A.  Scala  (I.e.),  among  others,  have 
plainly  expressed  themselves  in  favor  of  a  saturation  concentra- 
tion for  colloids.  The  method  employed  to  obtain  the  value  of 
the  critical  concentration  must  be  considered,  whether  this  be 
by  boiling  down  a  dilute  colloid  solution,  by  allowing  it  to 
evaporate  spontaneously,  by  removing  the  dispersing  medium 
through  nitration  (L.  Duclaux)  or  by  determining  the  maxi- 
mum spontaneous  colloid  solubility,  say  of  lead  (M.  Traube- 
Mengarini  and  A.  Scala,  I.e.),  or  congo-red  in  water  (W.  M. 
Bayliss,  I.e.),  etc.  The  different  methods  do  not  yield  the 
same  values,  as  J.  Duclaux  (I.e.)  has  pointed  out. 

While  experimental  evidence  seems  to  indicate  that  for 
suspensoids  there  is  an  upper  limit  beyond  which  the  system  is 
not  stable,  a  precise  determination  of  it  appears  to  be  connected 
with  serious  difficulties.  As  already  pointed  out,  the  saturation 
concentration  of  molecular  dispersoids  varies  with  mere  variations 
in  the  degree  of  dispersion  of  the  material  which  is  to  be  dis- 
solved as  shown,  for  example,  by  the  increased  solubility  of  a  sub- 
stance of  -molecule- disperse  solubility,  when  a  very  finely  ground 
powder  is  used.  One  may  accordingly  expect  highly  disperse 
colloids  to  have  a  higher  saturation  concentration  than  colloids 
of  a  lower  degree  of  dispersion.  A  statement  of  where  lies  the 
saturation  point  in  a  colloid,  just  as  a  statement  regarding  the 
concentration  of  a  colloid,  would  therefore  be  unambiguous 
only  if  the  degree  of  dispersion  of  the  colloid  could  be  stated  at 
the  same  time.  In  addition,  however,  this  circumstance  arises, 
that  the  disperse  phase  of  most  colloid  systems  contains  par- 
ticles of  different  dimensions.  The  instability  which  results 
from  this  will  be  discussed  later..  This  constitutes  a  further 
complication  in  the  definition  of  the  saturation  point  of  colloid 
systems. 

It  is  interesting  in  this  connection  to  bear  in  mind  that  coarsely 
disperse  systems  must  also  show  a  "saturation  concentration." 

1  J.  Duclaux,  Compt.  rend.,  148,  295  (1909);  Koll.-Zeitschr.,  7,  79  (1910). 

2  W.  M.  Bayliss,  Koll.-Zeitschr.,  6,  25  (1910). 


138  SPECIAL  COLLOID-CHEMISTRY 

Such  a  critical  concentration  must  appear  the  more  sharply  the 
more  uniform  the  degree  of  dispersion  of  the  disperse  phase.  The 
existence  of  such  critical  concentrations  in  coarse  emulsions  was 
first  pointed  out  by  Wa.  Ostwald.1  If  we  imagine  the  disperse 
phase  as  composed  of  equidimensional  spherical  drops,  the  highest 
attainable  concentration  will  be  reached  as  soon  as  the  drops 
just  touch  each  other  (without  suffering  deformation).  The 
dispersion  medium  then  only  fills  the  spaces  between  the  spherical 
drops.  Calculation  shows  that  for  spheres  thus  packed  together 
the  ratio  of  disperse  phase  to  dispersion  medium  is  as  74  is  to  26. 
Similar  considerations  apply  to  equidimensional  solid  disperse 
particles  of  regular  shape.  If  the  particles  are  of  unequal  size, 
in  other  words,  if  we  are  dealing  with  a  poly  dis  per  soid,  the  above 
does  not  apply,  since  the  smaller  particles  can  fill  the  spaces  be- 
tween the  larger  ones.  Under  such  circumstances  the  satura- 
tion point  would  depend  in  a  complicated  way  upon  the  relative 
proportions  of  the  differently  sized  particles  present.  The  above 
considerations  do  not  apply  at  all  when  the  size  of  the  disperse 
particles  changes  along  with  the  ratio  of  the  dispersion  medium  to 
the  disperse  phase,  as  in  the  typical  emulsoids,  or,  more  generally 
expressed,  in  concentration-variable  dispersoids. 

4.  Supersaturation  in  Colloid  Systems. — We  are  familiar  with 
the  fact  that  even  in  molecular  dispersoids  our  conception  of 
saturation  is  a  purely  relative  one.  This  is  evidenced  by  the 
existence  of  phenomena  of  super  saturation.  As  is  well  known,  a 
larger  quantity  of  material  yielding  a  molecularly  disperse  solu- 
tion can  be  dissolved,  if  particles  of  more  than  molecular  size  are 
excluded,  than  when  such  is  not  the  case.  But,  as  Wilh.  Ostwald2 
has  shown,  the  quantity  (and  therefore  the  size)  of  the  "  nucleus" 
which  is  thus  capable  of  suspending  the  state  of  supersaturation 
is  by  no  means  infinitely  small — it  is  not  even  so  small  as 
to  come  within  the  range  of  molecular  dimensions.  The  minimum 
quantity  of  sodium  chlorate  necessary  to  suspend  the  super- 
saturation  of  a  molecular  disperse  solution  of  the  same  salt  is 
equal  to  about  10  ~ 10  grams.  Small  as  this  quantity  is,  it  neverthe- 
less corresponds  to  a  cube  whose  sides  are  several  microns  long, 
so  that  it  is  too  large  to  fall  even  within  the  range  of  colloid  dis- 

1  Wa.  Ostwald,  Koll.-Zeitschr.,  6,  105  (1910);  see  also  M.  W.  Beyerinck,  ibid.,  7, 
16  (1910);  Wa.  Ostwald,  ibid.,  7,  64;  (1910)  E.  Hatschek,  ibid.,  7,  111^(1910). 

2  Wilh.  Ostwald,  Zeitschr.  f.  physik.  Chem.,  22,  289  (1897). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  139 

persoids.  Molecule-disperse  solutions  could  therefore  contain 
particles  of  more  than  molecular  dimensions  without  suspending 
the  state  of  supersaturation1  and  the  proportion  of  the  two 
phases  to  each  other,  in  other  words,  the  saturation  concen- 
tration could  vary  greatly  depending  upon  the  part  played  by 
the  non-molecular  phase.  That  such  mixed,  stable  systems  exist 
is  evidenced  by  the  fact  that  at  high  concentrations  many  molecu- 
lar dispersoids  assume  the  properties  of  colloid  systems.  Thus 
C.  A.  Lobry  de  Bruyn  and  L.  H.  Wolff2  have  shown  that  highly 
concentrated  cane-sugar  solutions  show  the  Tyndall  phenomenon. 
It  seems  not  impossible  that  an  " enlargement  of  the  molecules" 
or  polymerization  takes  place  in  high  concentrations  of  all 
molecular  dispersoids,  only  the  particles  rarely  attain  colloid 
dimensions.  Our  definition  of  concentration  and  saturation  in  all 
such  systems  requires  an  additional  clause  expressing  the  disper- 
sion of  the  system. 

But  there  exist  in  colloid  systems  also  an  interesting  series  of 
phenomena  which  can  be  satisfactorily  explained  only  if  we 
assume  the  possibility  of  supersaturation  in  them.  Thus  solu- 
tions of  gold  may  be  prepared  from  which  colloid  gold  separates 
spontaneously  in  high  concentrations,  but  in  which  this  does  not 
occur  or  only  after  prolonged  standing,  if  the  solution  is  more 
dilute.  R.  Zsigmondy,3  L.  Vanino  and  F.  Hartl,4  The  Svedberg,5 
Fr.  Doerinckel6  and  others  have  shown  that  the  formation  of 
colloid  gold  in  such  mixtures  may  be  accelerated  by  adding  a  few 
drops  of  a  previously  prepared  second  solution  of  colloid  gold. 
According  to  Zsigmondy  silver  sols  may  be  prepared  by  thus  "in- 
oculating" silver  solutions  with  colloid  particles.  Similar  super- 
saturation  phenomena  occur  in  the  silver  sols  of  the  photographic 
plate,  according  to  Llippo-Cramer.7  It  is  of  great  interest  that  such 
phenomena  are  encountered  among  emulsoids.  Thus  H.  Garrett8 
found  that  gelatine  solutions  congeal  more  rapidly  if  some  solid 

1  See  also  in  this  connection  the  numerous  papers  by  P.  P.  von  Weimarn  in  the 
Kolloid-Zeitschrift  and  the  Kolloid-chemische  Beihefte. 

2  C.  A.  Lobry  de  Bruyn  und  L.  H.  Wolff,  Rec.  trav.  chim.  desPays.  Bas.,23, 155 
(1904). 

3  R.  Zsigmondy,  Z.  f.  physik.  Chem.,  56,  65,  57  (1906). 

4L.  Vanino  und  F.  Hartl,  Ber.  d.  Dtsch.  chem.  Ges.,  39,  1699  (1906). 

5  The  Svedberg,  Koll.-Zeitschr.,  6,  238  (1910). 

6  Fr.  Doerinckel,  Z.  f.  anorg.  Chem.,  63,  344  (1909). 
7Luppo-Cramer,  Koll.-Zeitschr.,  7,  99  (1910). 

8  H.  Garrett,  Uber  d.  Viskositat  einiger  Kolloid-Losungen  usw.  Diss.  Heidel- 
berg, 1903;  Philos.  Mag.  [6],  6,  374  (1903). 


140  SPECIAL  COLLOID-CHEMISTRY 

gelatine  prepared  by  rapid  cooling  of  the  same  solution  is  added 
to  it.  F.  EduardofT1  has  noted  that  the  natural  emulsions  of 
rubber  from  certain  rubber  plants,  which  undergo  "spontaneous" 
coagulation  when  exposed  to  the  air,  coagulate  more  rapidly  if  a 
piece  of  solid  (i.e.,  already  coagulated)  rubber  is  introduced  into 
the  liquid.  Eduardoff  believes  that  he  has  excluded  the  possi- 
bilities of  a  chemical  action  in  this  illustration.  The  experiments 
of  W.  Biltz  and  A.  von  Vegesack2  should  also  be  cited  here. 
They  found  that  in  emulsoid  night-blue  hydrosol  the  increase  in 
viscosity  due  to  ageing  is  accelerated  by  "inoculation"  with  small 
amounts  of  already  viscid  night- blue  (for  details  see  §25). 

The  view  that  all  these  phenomena  are  due  to  the  suspending 
of  a  state  of  supersaturation,  which  gives  rise  to  coarsely  disperse 
systems,  cannot  be  doubted. 

§24.  Molecular  Weight  of  Substances  in  the  Colloid  State  as 
Measured  by  Changes  in  the  Constants  of  the  Dispersing 

Medium 

i.  General  Remarks. — The  quantitative  relations  which  exist 
between  concentration  and  the  lowering  of  the  vapor  pressure,  the 
elevation  of  the  boiling  point,  or  the  depression  of  the  freezing 
point  of  molecularly  disperse  solutions,  allow  of  the  determination 
of  molecular  weight.  Such  determinations  must  evidently  be- 
come more  difficult  and  less  reliable  as  the  amount  of  such  change 
becomes  smaller.  As  shown  in  §22  these  changes  almost  dis- 
appear in  dispersoids  of  medium  or  low  dispersion,  such  as  the 
colloids.  A  source  of  great  error  in  these  investigations  is  the 
molecularly  dispersed  impurities  present  in  such  colloids  which 
can  be  removed  only  with  greatest  difficulty.  Experiments 
have  been  made  in  which  the  effect  has  been  determined  of  con- 
centration of  the  colloid  on  the  lowering  of  the  freezing  point 
and  the  elevation  of  the  boiling  point  of  the  dispersing  medium 
and  no  change  has  been  noted. 

To  this  must  be  added  that  in  the  cases  in  which  a  change  in 
freezing  point,  boiling  point,  etc.,  with  change  in  concentration  of 
colloid  has  been  observed,  the  amount  of  such  change  has  not  been 
even  approximately  proportional  to  the  change  in  concentration, 

1F.  Eduardoff,  Gummi-Ztg.,  23,  809  (1909). 

2  W.  Biltz  und  A.  von  Vegesack,  Z.  f.  physik.  Chem.,  73,  509  (1910). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS      141 

as  the  previously  cited  investigations  of  G.  Tammann  and  D. 
Kanowalow  show.  The  recent  investigations  of  T.  B.  Robertson 
and  Th.  Burnett1  of  the  freezing  point  of  casein  in  the  presence 
of  m/3O  KOH  show  that  this  remains  constant,  even  when  the 
casein  content  of  the  solution  varies  to  the  extent  of  50  per  cent. 
In  colloid  solutions  not  even  the  sense  may  be  preserved  of  the 
relations  between  concentration  and  changes  in  these  properties. 
Thus,  as  shown  on  p.  130,  the  elevation  of  the  boiling  point  as 
well  as  the  lowering  of  the  vapor  pressure  instead  of  increasing 
may  decrease  relatively,  or  even  absolutely,  in  soap  solutions 
with  increase  in  concentration.  The  changes  in  state  which  are 
responsible  for  this  behavior  are  not  limited  to  this  example, 
but  doubtlessly  occur  in  solutions  of  egg-albumin  when  the  relative 
proportions  of  disperse  phase  and  dispersing  medium  are  changed. 
Finally,  it  should  be  emphasized  that  it  is  possible  to  prepare 
systems  having  the  same  chemical  composition  but  progressively 
varying  degrees  of  dispersion  (see  for  example  p.  143).  In 
view  of  these  facts  a  discussion  of  the  "  molecular  weights  of 
colloid  substances "  loses  all  significance.  Otherwise  an  in- 
vestigation of  the  aqueous  arsenious  trisulphide  solutions  of 
Linder  and  Picton,  would  show  a  series  of  progressively  varying 
" molecular  weights"  for  one  and  the  same  chemical  substance  in 
the  same  solvent,  which  in  the  coarser  suspensions  would  approach 
infinity.  The  interest  attached  to  a  quantitative  or  graphic 
determination  of  the  relations  between  "molecular  weights" 
and  the  size  of  the  particles,  for  a  general  theory  of  the  dispersoid 
state,  scarcely  requires  emphasis. 

It  follows  from  the  above  that  one  cannot  properly  speak  of 
the  molecular  weights  of  dispersoids  having  a  colloid  or  lower 
degree  of  dispersion  as  one  does  of  the  molecular  weights  of 
molecular  dispersoids  as  deduced  from  changes  in  the  constants 
of  their  solvents.  Measurements  of  the  vapor  pressure,  the  boil- 
ing point  and  the  freezing  point  of  colloid  systems  certainly  do 
not  justify  it.  This  needs  to  be  emphasized,  for  only  recently  in- 
vestigators like  S.  Arrhenius,2  T.  B.  Robertson,  etc.,  have  based 
chemical  conclusions  upon  the  "molecular  weights"  of  such  colloid 
systems  as  egg-albumin  without  paying  attention  to  the  variabil- 

1T.  B.  Robertson  and  Th.  Burnett,  Jour.  Biol.  Chem.,  6,  105  (1909). 
2  S.  Arrhenius:  Immunochemie,  16,  19,  24,  etc.,  Leipzig,  1907. 


142  SPECIAL  COLLOID-CHEMISTRY 

ity  in  degree  of  dispersion  with  changes  in  concentration.  It 
cannot  be  emphasized  too  strongly  that  not  even  the  direction  of 
the  changes  in  solution  constants  with  changes  in  concentration 
need  be  the  same  in  the  case  of  colloids,  as  shown  by  the  soap 
solutions  referred  to  on  p.  129;  much  less  does  there  exist  a  propor- 
tionality between  such  changes  and  concentration  as  the  laws  of 
van't  Hoff  demand.  For  the  same  reason  the  mass  law  of  chem- 
ical reactions1  is  not  applicable  to  colloid  systems,  for  (in  its  ordi- 
nary form)  this  assumes  a  proportionality  between  molecular 
concentration  and  concentration  by  weight. 

2.  Examples  of  the  "Molecular  Weights"  of  Substances  in  the 
Colloid  State  as  determined  by  Changes  in  the  Constants  of  the 
Dispersing  Medium. — Even  though  the  value  of  determining 
the  "molecular  weight"  of  colloid  substances  is  a  rather  doubtful 
one  after  what  we  have  said,  we  shall  nevertheless  give  a  few 
examples  as  determined  from  changes  in  vapor  pressure,  boiling 
point  and  freezing  point. 

Of  older  determinations  may  be  mentioned  those  of  J.  H. 
Gladstone  and  W.  Hibbert2  who  found  that  purified  egg-albumin 
shows  an  unmeasurably  slight  depression  of  the  freezing  point. 
The  same  authors  found  caramel  to  show  a  molecular  weight 
of  1585  to  1745,  gum  arabic,  one  of  1612  to  2001;  rubber  in  ben- 
zene, one  of  6504.  Later  measurements  by  the  same  method, 
carried  out  by  St.  Burgarsky  and  L.  Liebermann  (I.e.),  gave  for 
egg-albumin  a  molecular  weight  of  6400,  for  albumose,  2400, 
for  pepsin,  760.  Sabenejew  and  Alexandrow  (I.e.)  find  egg- 
albumin  to  have  a  molecular  weight  of  14270.  From  freezing- 
point  determinations  T.  B.  Robertson  and  Th.  Burnett  (I.e.) 
assign  a  molecular  weight  of  1400  to  2000  to  casein  in  the  presence 
of  bases.  Z.  Gatin-Gruszewska  (I.e.)  finds  highly  purified 
glycogen  to  have  a  "molecular  weight"  of  140,000  or  more. 

Of  measurements  of  the  boiling  point  of  colloid  systems  those 
of  F.  Krafft  and  his  pupils  (I.e.)  deserve  special  mention.  The 
following  table  shows  a  particularly  interesting  series  of  their 
experiments.  It  shows  at  the  same  time  that  in  the  lower  mem- 
bers of  a  homologous  series  the  observed  molecular  weights  in 
aqueous  solution  are  at  first  lower  than  calculated,  while  the 

i  See  in  this  connection  the  textbooks  of  physical  chemistry;  for  example,  Wilh. 
Ostwald,  Grundr.  d.  allg.  Chem.  4Aufl.,  341,  375,  etc.,  Leipzig,  1909. 
*  For  literature  see  p.  129. 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


143 


reverse  is  true  with  the  higher  members.  We  deal,  in  other  words, 
with  a  series  of  dispersoids  which  vary  in  the  degrees  of  their 
dispersion.  Dispersion  values  characteristic  of  colloids  begin,, 
approximately,  with  nonylate. 

TABLE  n. — MOLECULAR  WEIGHTS  OF  AQUEOUS  SODIUM-SOAP  SOLUTIONS 
(Determined  by  Boiling-point  Measurements.)    After  F.  Krafft  and  A.  Strutz1 


Concentration  in 

Molecula 

r  weight 

Sodium  salt 

per  cent. 

Observed 

Calculated     . 

Acetate,  C2H3O2  

O.Q 

50.5 

82 

25.2 

40.3 

Propionate,  CaHsC^  

3-8 

SI.7 

96 

19.8 

42.6 

Capronate,  CeHuOs  

3-5 

72.8 

138 

20.6 

77-9 

Nonylate,  C9Hi7O2  

3-4 

144.1 

180 

20.4 

285.5 

Laureate,  C23H2202  

3-3 

474.0 

222 

16.1 

507.0 

Palmitate,  C16H31O2  

16.4 

about  1060 

278 

25.0 

approaches  <» 

Stearate,  Ci8Hi8O2  -  

about  1  6.0 

about  1500 

306 

27.0 

approaches  oo 

(Oleate,  CisHssOs) 

27    O 

. 

•?O4 

*•  /  .  \j 
26.5 

approaches  oo 

o   ^ 

Of  "molecular  weight  determinations"  in  colloid  systems 
obtained  by  measuring  the  vapor  pressure  those  of  H.  Rodewald 
(I.e.)  must  be  mentioned.  From  study  of  a  starch- water  system 
containing  68.37  Per  cent,  of  starch  by  weight  Rodewald  calculated 
this  to  have  the  unexpectedly  low  molecular  weight  of  4370. 

It  must  be  emphasized  that  a  great  number  of  observations 
of  an  entirely  negative  character  stand  out  against  the  figures 
given  above,  and  that  it  has  been  found  generally  true,  as  in  the 
observations  of  Z.  Gatin-Gruszewska  (I.e.),  that  the  molecular 
weights  become  progressively  greater  with  increase  in  the  purity 
of  the  colloid  employed.  It  has  been  maintained  by  some  authors 
(see,  for  example,  T.  B.  Robertson,  I.e.)  that  such  purification, 
say  of  an  aqueous  colloid,  is  fundamentally  wrong  because  the 
impurities  represent  integral  parts  of  the  colloid  phase  and  should 

1  F.  Krafft  und  A.  Strutz,  Ber.  d.  Dtsch.  chem.  Ges.,  29,  1329  (1896). 


144  SPECIAL  COLLOID-CHEMISTRY 

therefore  be  counted  in  with  it  as  part  of  its  "  chemical  constitu- 
tion." It  must,  of  course,  be  admitted  that  proteins,  such  as 
casein  for  example,  can  form  salt-like  compounds  with  electro- 
lytes, though  it  is  questionable  whether  we  are  in  such  cases 
really  dealing  with  stoichiometrical  relations  or  only  "  adsorption 
compounds."  Salts  of  protein,  especially  salt-like  compounds  of 
the  products  of  protein  cleavage,  do  undoubtedly  show  measur- 
able molecular  weights  (see  p.  132);  but  that  fact  does  not  yet 
do  away  with  the  question  of  the  "molecular  weight"  of  pure 
proteins,  as  these  observers  apparently  fail  to  see.  That  electro- 
lytes need  not  form  an  integral  part  of  colloid  systems  is  shown  by 
the  example  of  glycogen  cited  above  and  by  such  compounds  as 
metastyrol1  which  also  induces  no  measurable  increase  in  boiling 
point.  The  "explanation"  of  these  facts  as  given  by  these 
authors,  according  to  which  there  occurs  a  great  "polymerization" 
of  the  protein  when  pure,  resulting  in  an  abnormal  molecular  weigh  t 
is  only  a  restatement  in  chemical  terms  of  the  physical  fact  that 
the  degree  of  dispersion  of  a  pure  protein  solution  has  assumed  the 
values  characteristic  of  colloid  systems. 

Parenthetically,  it  is  well  to  point  out  even  here  that  similarly 
high  values  have  been  found  when  the  "molecular  weights"  have 
been  determined  by  other  means.  It  must  be  acknowledged  that 
there  exist  considerable  discrepancies  between  the  values  obtained 
by  the  methods  here  discussed  and  by  the  "dynamic"  methods  of 
determining  the  molecular  weight  (as  by  direct  measurement  of  the 
osmotic  pressure,  diffusion  velocity,  etc.).  Protein  solutions,  for 
example,  exhibit  a  direct  osmotic  pressure,  while  they  show  no 
measurable  increase  in  the  boiling  point.2  Even  these  facts  are 
sufficient  to  show  that  a  careless  application  of  van't  HofFs 
laws  of  solution  is  not  to  be  made  to  colloid  systems,  for  the 
necessary  quantitative  relationships  are  lacking.  For  this  reason 
it  seems  better  to  return  to  mere  description  of  the  individual 
groups  of  phenomena  observed  in  colloid  systems  and  so  obtain 
laws  by  inductive  methods  which  will,  perhaps,  be  analogous  to 
but  not  identical  with  those  of  "true"  solutions. 

1  G.  Posnjak,  Das  Metastyrol  usw.,  15,  Diss.,  Leipzig,  1910. 

2  See,  for  example,  R.  S.  Lillie,  Amer.  Jour.  Physiol.,  20,  127  (1907).     Another 
coarse  discrepancy  was  pointed  out  above  in  discussing  the  behavior  of  soap  solu- 
tions. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  145 

H.    INTERNAL  FRICTION  AND  SURFACE  TENSION  OF 

COLLOIDS 

§25.  Internal  Friction  of  Colloid  Systems 

i.  General  Remarks. — The  internal  friction  or  viscosity  of 
colloids  has  not  yet  been  as  thoroughly  investigated  as  the 
viscosity  of  molecular  dispersoids.  Yet  this  very  property 
demands  special  study  in  the  case  of  colloids.  Even  slight  changes 
in  colloids,  particularly  in  the  case  of  the  emulsoids,  so  greatly 
affect  this  value  that  even  Thomas  Graham  justly  called  the 
viscosimeter  a  "colloidoscope."  Such  viscosity  measurements 
can,  moreover,  be  easily  made.1  It  is  therefore  rather  strange 
that  the  attention  of  colloid  investigators  has  only  recently  been 
directed  to  this  problem. 

The  following  are  the  main  characteristics  of  molecular  dis- 
persoids : 

When  a  solid  goes  into  solution  the  viscosity  of  the  dispersing 
medium  is  usually  increased.  Dilute  solutions  of  some  salts, 
such  as  lithium  chloride  and  potassium  chloride,  form  exceptions 
to  this  rule.  These  represent  cases  of  so-called  negative  viscosity. 
When  solid  naphthaline  is  dissolved  at  room  temperature  in 
alcohol  a  decrease  in  viscosity  is  also  observed,  according  to  experi- 
ments of  my  own.  We  here  apparently  deal  with  iso-dispersoid 
solvents  which  are  "  depolymerized "  when  the  given  substances 
are  dissolved  in  them.  In  normal  cases  the  viscosity  increases 
progressively  with  concentration,  but  more  rapidly  than  the 
latter,  so  that  the  viscosity-concentration  curve  is  convex  toward 
the  concentration  axis. 

Manifold  and  complicated  conditions  arise  when  two  liquids 
are  molecularly  dissolved  in  each  other.  Thus,  the  mixture  of 
alcohol  with  water  shows  a  characteristic  behavior  in  that  a  maxi- 
mum of  viscosity  is  obtained  in  medium  concentrations  of  the 
one  in  the  other,  the  value  of  which  is  considerably  greater  than 
that  of  the  pure  components.  For  further  details  the  reader  must 

1  The  simplest  apparatus  yielding  the  most  accurate  results  is  the  viscosimeter  of 
Wilhelm  Ostwald  consisting  of  a  U-shaped  tube  in  which  is  measured  the  time  of 
outflow  of  a  constant  volume  through  a  capillary.  The  so-called  relative  viscosity  is 
proportional  to  the  time  of  outflow  and  the  viscosity  of  the  liquid.  For  details 
regarding  the  method  see  Ostwald-Luther-Drucker,  Handbuch  f.  Physik.-chem. 
Messungen.,  3,  230,  Leipzig,  1910. 
10 


146  SPECIAL  COLLOID-CHEMISTRY 

be  referred  elsewhere.1  As  already  evident,  this  behavior  cor- 
responds closely  with  the  extremely  variable  viscosity  observed  in 
emulsoids. 

When  gases  are  dissolved  in  a  solvent  they  do  not  seem  to 
change  its  viscosity.2 

2.  Internal  Friction  of  Suspensoids. — It  may  be  regarded  as 
typical  of  suspensoids  that  their  viscosity  is  but  slightly  greater  than 
that  of  their  pure  dispersing  mediums.  It  must  be  remembered, 
however,  that  this  is  true  only  when  such  systems  are  dilute 
(see  p.  135).  In  concentrated  form  the  mass  of  the  disperse 
solid  phase  may  predominate  over  that  of  the  dispersing  medium, 
as  when  powders  are  merely  moistened  so  as  to  be  coated  by  a  thin 
but  continuous  liquid  membrane.  Such  systems  may  be  so  viscid 
that  their  properties  approximate  those  of  solids.  We  need  but 
recall  how  moist  sand  may  be  cut  into  slices,  and  the  rigidity  of  the 
scales  and  crusts  of  dried  colloid  metals.  From  this  it  follows 
that  with  increase  in  concentration  the  viscosity  of  a  suspensoid 
rises  very  slowly  at  first,  but  very  suddenly  and  greatly  in  high 
concentrations. 

Theoretically  we  may  anticipate  finding  two  concentration 
regions  in  which  there  occurs  a  relatively  rapid  rise  of  viscosity  in 
suspensoids.  We  have  frequently  made  mention  of  the  fact  (see 
p.  86)  that  moistened  bodies  are  coated  with  a  liquid  film  which 
adheres  to  them  and  which  differs  in  many  respects  from  the 
remaining  "free"  liquid.  In  the  case  of  molecular  dispersoids 
these  complexes  of  disperse  phase  with  "fixed"  solvent  are 
called  sohates.  Analogous  phenomena  must  be  assumed  to  occur 
whenever  a  disperse  phase  is  moistened  by  a  dispersing  medium. 
When  the  disperse  particles  move,  these  liquid  envelopes  must 
evidently  be  dragged  along  and  thus  movement  be  retarded. 
With  progressive  increase  in  the  thickness  of  these  liquid  envelopes 
a  middle  concentration  can  readily  be  imagined  in  which  the 
individual  layers  are  independent  of  each  other,  but  nevertheless 
retard  their  mutual  mobility  through  frequent  impacts  with  each 
other.  Though  the  particles  may  still  glide  past  each  other  as 
they  do  in  viscosity  measurements,  say  in  a  capillary  viscosimeter, 

1  A  recent  review  with  original  experiments  is  found  in  the  dissertation  of  R. 
Kassel,  Viskositat  binarer  Fiussigkeitsgemische,  Leipzig,  1910. 

2  At  least  no  changes  were  observed  with  the  ordinary  gases  (O,  N,  CO2,  CH4). 
See  Wo.  Ostwald  and  A.  Genthe,  Zool.  Jahrb.,  Abth.  f.  Biol.,  18,  12  (1903). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  147 

a  greater  length  of  time  is  demanded.  This  first  concentration 
region  lies  far  below  the  second  concentration  region  which  we 
discussed  previously.  The  distinguishing  characteristic  between 
the  two  is  found  in  the  fact  that  in  the  case  just  discussed  the 
liquid  films  are  still  independent  of  each  other,  while  in  the  highly 
concentrated  colloid  (say  one  containing  99  per  cent,  solid)  the 
liquid  films  must  be  regarded  as  having  coalesced  and  so  being 
continuous. 

Most  viscosity  measurements  of  suspensoids  have  been  made 
on  dilute  preparations  as  by  J.  Friedlander,1  F.  Bottazzi  and 
G.  d'Errico,2  on  glycogen  solutions,  and  by  H.  W.  Woudstra3  and 
others  on  silver  hydrosols.  Measurements  carried  out  on  transition 
systems  such  as  iron  hydroxide  sols  will  be  discussed  below.  It 
needs  to  be  emphasized  that  glycogen  solutions  are  typical  sus- 
pensoids only  in  low  concentrations,  and  that  they  assume  an 
emulsoid  character  in  higher  ones.  This  is  already  evidenced  by 
the  fact  that  glycogen  sols  may  be  prepared  which  contain  26  to 
45  per  cent,  of  glycogen.4  Nevertheless  in  low  concentrations 
glycogen  behaves  as  a  typical  suspensoid,  and  so  its  discussion 
seems  appropriate  in  this  place. 

The  viscosity  of  suspensoids  appears  to  have  been  measured 
for  the  first  time  by  J.  Friedlander  (I.e.)  who  could  scarcely 
detect  a  difference  between  them  and  their  pure  dispersing 
mediums.  For  a  suspension  of  rosin  (10  cc.  of  a  i  per  cent,  alco- 
holic solution  squirted  into  150  cc.  of  water)  he  obtained  a 
viscosity  of  598.4;  after  43  hours  it  had  assumed  a  viscosity  of 
599.3.  Pure  water  has  a  value  of  599.6.  A  "very  turbid"  silver 
sol  (Ag-Crede)  gave  453.4  while  pure  water  gave  452.3.  These 
differences  are  scarcely  greater  than  the  experimental  error. 
Even  though  these  measurements  refer  to  very  dilute  systems,  they 
suffice  to  show  how  very  slight  are  the  changes  in  the  viscosity  of 
a  liquid  when  it  takes  up  a  suspensoid  phase. 

More  detailed  work  was  then  done  by  F.  Bottazzi  and  G. 
d'Errico  (I.e.)  as  well  as  by  H.  W.  Woudstra  (I.e.).  Their  more 

1  J.  Friedlander,  Zeitschr.  f.  physik.  Chem.,  38,  430  (1901). 

2  F.  Bottazzi  und  G.  d'Errico,  Pfliiger's  Arch.  f.  Physiol.,  115,  359  (1906). 
3H.  W.  Woudstra,  Z.  f.  physik.  Chem.,  63,  619  (1908);  Chem.  Weekblad,  5, 

303  (1908);  _van  Bemmelen-Gedenkboek,  36  (1910). 

4  According  to  Z.  Gatin-Gruszewska,  Pfliiger's  Arch.,  102,  569  (1904)  only"  20 
per  cent,  solutions  of  pure  glycogen  can  be  prepared;  if  the  glycogen  contains  salt 
its  solubility  is  much  greater. 


148 


SPECIAL  COLLOID-CHEMISTRY 


important  results  are  given  in  Table  12  and  in  Fig.  22,  to  which 
have  been  added  some  data  on  sodium  chloride  obtained  by  A. 
Genthe  and  myself. 


TABLE  12. — VISCOSITY  OF  SUSPENSOIDS 


Silver  hydrosol  (according  to 

H.  W.  Woudstra). 

Temp.  26° 


Glycogen  hydrosol  (according 

to  F.  Bottazzi  and  G. 

d'Errico). 

Temp.  37° 


NaCl  (for  comparison),  (ac- 
cording to  Wo.  Ostwald  and 
A.  Genth'e).i 
Temp.  20° 


Concentration, 
per  cent. 

Viscosity 

Concentra- 
tion, 
per  cent. 

Viscosity  (time 
of  outflow), 
sec. 

Concentra- 
tion, 
per  cent. 

Viscosity  A, 
sec. 

j 

0  .  0000 

.0000 

o 

124    ) 

O 

56-2     1 

0.9310 

.0013 

i 

129     f     33 

I 

56.58   \    4.0 

1.9025 

.0021 

5 

157    j 

10 

60.  21   J 

2.887 

.0045 

10 

208           51 

15 

65-95       5-8 

3.369 

.0057 

ID 

259           5i 

20 

75.24      9.2 

3.850 

I  .  0098 

20 

440         191 

25 

87.44       12.2 

4.904 

1-0457 

25 

S64*(?)i24 

26.52 

103.63     16.2 

30 

914         350 

IIO-7 

35 

1516         602 

- 

40 

3549       2033 

45 

7688       4139 

*  A  second  viscosimeter  was  used  from  this  point  on,  the  outflow  time  of  which 
compared  with  the  first  as  i  :  2.6.  All  the  values  have  been  recalculated  in  terms 
of  the  first. 

The  curves  and  tables  show  that  at  certain  concentrations 
there  is  a  very  sudden  increase  in  viscosity.  For  silver  and  glyco- 
gen  hydrosols  these  concentrations  are  respectively  about  3.5 
and  30  per  cent.  The  almost  rectilinear  character  of  the 
first  part  of  the  curves  shows  that  for  low  concentrations  the 
increase  in  viscosity  is  almost  directly  proportional  to  the  colloid 
content.  When  these  curves  are  compared  with  that  for  NaCl 
(o  to  25.52  per  cent.),  one  notes  a  straighter  curve.  The  uniformity 
of  the  NaCl  curve  is  still  greater  at  higher  temperatures,  so  that  at 
a  temperature  corresponding  to  that  at  which  the  measurements 
on  the  colloids  were  carried  out  it  would  approximate  a  straight 
line. 

The  very  slight  absolute  increase  in  viscosity  at  low  colloid 
concentrations  should  be  emphasized.  Silver  hydrosol  containing 
3.85  per  cent,  colloid  silver  is  less  than  i  per  cent,  more  viscid 
than  pure  water,  while  a  5  per  cent,  solution  of  NaCl  shows  an 

1  Wo.  Ostwald  and  A.  Genthe,  Zool.  Jahrb.,  Abt.  f.  Biol.,  18,  i  (1903). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


149. 


increase  of  6.6  per  cent.  Glycogen  shows  a  greater  absolute 
increase  in  viscosity  (about  4  per  cent,  for  a  i  per  cent,  glycogen 
content) .  Still,  molecular  dispersoids  are  also  known  which  yield 
such  high  figures,  as  in  the  case  of  sugar  in  water.  Furthermore, 
as  already  mentioned,  glycogen  solutions  are  not  to  be  regarded 
as  typical  suspensoids. 


FIG.  22. — Viscosity  of  suspensoids.     (According  to  H.  W.  Woudstra,  F.  Botazzi  and 
G.  d'Errico.)     The  curve  for  NaCl  has  been  added  for  purposes  of  comparison. 

So  far  as  the  relation  is  concerned  of  the  sudden  increase  in 
viscosity  here  noted  to  the  critical  concentrations  previously 
discussed,  it  may  be  said  that  the  behavior  of  silver  hydrosol  at 
the  3.5  per  cent,  concentration  coincides  with  the  first  of  these 
critical  regions.  It  is  interesting,  and  incidentally  confirms  the 
idea  that  dilute  glycogen  solutions  have  a  suspensoid  character, 
that  F.  Bottazzi  and  G.  d'Errico  (I.e.)  also  emphasize  the  slight- 


SPECIAL  COLLOID-CHEMISTRY 


ness  of  increase  in  viscosity  that  can  be  observed.  On  the  scale 
chosen  for  Fig.  22  this  increase  is  scarcely  apparent.  The  figures 
of  Table  12  show  clearly,  however,  a  decided  increase  between 
the  concentrations  of  15  to  20  per  cent.  The  steepening  of  the 
curve  in  Fig.  22  at  45  per  cent,  evidently  indicated  an  approach 
to  the  second  concentration  region  where  the  system  begins  to 
assume  the  viscosity  of  solid  bodies,  although  one  must  consider 
that  at  this  point  the  suspensoid  character  of  the  system  is 
probably  beginning  to  make  way  for  a  more  emulsoid  one. 

3.  Effects  of  External  Conditions  upon  Viscosity  of  Suspen- 
soids. — Temperature  affects  the  viscosity  of  suspensoids  in  the  same 
way  as  it  does  that  of  normal  liquids :  the  viscosity  decreases  with 
increasing  temperature.  There  is  nothing  especially  peculiar 
about  the  viscosity  of  suspensoids  when  compared  with  that  of 
molecular  dispersoids  or  that  of  pure  dispersion  media.  The  fact, 
however,  that  the  viscosity  of  suspensoids  is  thermostable,  in  other 
words,  that  it  is  the  same  for  a  given  temperature  whether  one 
starts  from  a  higher  or  a  lower  one  is  very  important.  The  pre- 
vious thermal  history  does  not  influence  the  viscosity  of  suspen- 
soid colloids  any  more  than  it  does  that  of  molecular  dispersoids. 
This  is  indicated  by  the  experiments  of  F.  Bottazzi  and  G.  d'Errico 
(I.e.)  on  glycogen  solutions  as  reproduced  in  the  following  table. 

TABLE  13. — INFLUENCE  OF  THERMAL  HISTORY  ON  VISCOSITY  OF  A  10  PER  CENT. 

SOLUTION  OF  GLYCOGEN 
(According  to  F.  Bottazzi  and  G.  d'Errico) 


Temperature 

Time  of  outflow  in  seconds  after 

Heating 

Cooling 

A 

70° 
60 

So 
40 

1  2O 

137 
157 
189 

123 
143 
164 
191 

-3 
-6 

-7 

—  2 

3°  . 
20' 

234 
284 

233 
286 

—  2 

Although  the  deviations  all  tend  toward  the  same  side  they 
scarcely  exceed  the  limits  of  experimental  error,  particularly  if  one 
considers  that  the  slightly  greater  values  may  have  been  due  to 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS      151 

evaporation,  for  the  measurements  on  the  cooled  solutions  were 
made  later  than  those  on  the  warmed  solutions. 

This  behavior  of  the  suspensoids  stands  in  marked  contrast 
to  that  of  the  emulsoids  as  will  appear  below.  The  suspensoids 
tend  here  to  behave  like  the  molecular  dispersoids  which  is  rather 
unusual,  for  ordinarily  the  emulsoids  do  this. 

According  to  H.  W.  Woudstra  (I.e.)  the  addition  of  electrolytes 
influences  the  viscosity  of  a  suspensoid  silver  hydrosol  (and  prob- 
ably that  of  all  suspensoids)  in  a  very  characteristic  manner.  The 
addition  of  an  electrolyte  decreases  the  viscosity.  To  judge  from 
the  behavior  of  molecular  dispersoids  one  would  expect  an  increase 
in  viscosity  upon  adding  an  electrolyte  but  actually  the  opposite 
occurs.  It  should  be  noted,  however,  that  this  decrease  takes 
time,  occurring  in  some  instances  only  after  days,  and  that  it  is 
associated  with  a  change  in  the  state  of  the  colloid,  namely,  with 
its  coagulation.  The  following  table  shows  this  behavior. 


TABLE  14. — INFLUENCE  or  ELECTROLYTES  ON  THE  VISCOSITY  OF  SILVER 

HYDROSOL 

(According  to  H.  W.  Woudstra) 
28  cc.  silversol  +  i  cc.  K2SO4  solution  =  0.015  millimol. 


Time  in  days 

Viscosity 

After    4  days  . 

.O";o7 

After  1  8  days 

0126 

After  34  days  

.0047 

After  36  days* 

oo88(?) 

After  47  days* 

OOA3 

*  The  asterisk  signifies  that  silver  has  already  begun  to  precipitate. 


The  influence  of  age  on  the  viscosity  of  a  suspensoid  is  closely 
connected  with  the  above.  H.  W.  Woudstra  (I.e.)  found  that 
silver  sols  gradually  become  less  viscous  even  when  nothing  is  added 
to  them.  This  is  explained  by  the  fact  that  in  the  preparation  of 
most  inorganic  colloids  small  quantities  of  electrolytes  are  retained 
by  the  colloids,  and  these  tend  to  bring  about  coagulation.  Table 
15  may  serve  as  an  example. 


152  SPECIAL  COLLOID-CHEMISTRY 

TABLE  15. — INFLUENCE  OF  AGE  ON  THE  VISCOSITY  OF  A  SILVER  HYDROSOL 
(According  to  H.  W.  Woudstra) 

i 
Age  in  days  Viscosity 


3 

1-0457 

17 

i  .0201 

28^ 

i  .0107 

37 

52* 

1.0077 
i.on8(?) 

*  Silver  has  begun  to  precipitate;  the  higher  viscosity  value  may  perhaps  be  ex- 
plained by  clogging  of  the  capillary. 

We  must  keep  in  mind  that  the  action  of  electrolytes  and  of  age 
in  decreasing  the  viscosity  of  suspensoids  is  the  direct  opposite 
of  their  effect  on  the  viscosity  of  most  emulsoids,  as  will  be  dis- 
cussed below. 

4.  Mechanical  Theory  of  the  Viscosity  Relations  in  Suspen- 
soids. —  E.  Hatschek1  working  from  a  physical  point  of  view  has  re- 
cently undertaken  to  reduce  the  viscosity  relations  of  dispersoids  to 
mathematical  terms.  For  the  details  of  the  theoretical  argument 
the  original  must  be  consulted;  of  his  conclusions  the  following 
seem  particularly  important. 

The  increase  in  viscosity  of  a  liquid  upon  addition  of  a  disperse 
phase  is  directly  proportional  to  the  percentage  of  solid  substance 
added,  but  is  independent  of  the  degree  of  dispersion  and  of  the 
distance  between  the  individual  disperse  particles.  The  relation 
may  be  expressed  by  the  formula: 


in  which  t]'  is  the  viscosity  of  the  suspensoid,  rj,  that  of  the  pure 

volume  of  the  solid  substance 

dispersion  means,  and/,  the  relation™  —  —  —  -  —  -  • 

total  volume 

To  get  an  idea  of  the  numerical  values  involved  let  us  choose  a  sus- 
pensoid containing  10  per  cent,  solid  substance.  In  this  case  the 
viscosity  of  the  dispersoid  rises  to  1.45  times  that  of  the  dispersion 
means  .  The  maximal  viscosity  which  a  suspensoid  can  have  accord- 
ing to  this  formula  amounts  to  2.8  times  that  of  the  pure  disper- 
sion means.  Clearly,  these  theoretical  results  agree  throughout 
with  practical  experience  for  they  predict  only  a  small  increase 

1  E.  Hatschek,  Koll.-Zeitschr.,  7,  301  (1910);  studies  on  the  viscosity  of  emulsoids 
are  under  way. 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS      153 

in  viscosity  in  suspensoids.  But  more  study  seems  necessary 
for  a  complete  formulation  of  the  viscosity  relations  in  suspen- 
soids by  the  deductive  method  which  E.  Hatschek  has  begun 
in  such  a  promising  way.  Hatschek's  theoretical  conclusion 
that  the  degree  of  dispersion  of  a  suspensoid  has  no  effect  on 
its  viscosity  seems  unconvincing,  for  a  relationship  between  the 
two  has  been  observed  in  emulsoids  (see  below). 

5.  Viscosity  of  Emulsoids  (Literature). — A  comparison  of 
the  viscosity  of  emulsoids  and  coarser  emulsions  with  that  of 
suspensoids  has  recently  been  the  object  of  thorough  study. 
From  the  great  literature  on  this  subject  we  may  mention  the 
papers  of  P.  von  Schroeder1  (gelatine),  V.  Henri,  Lalou,  A. 
Mayer,  Stodel,2  H.  Garrett3  (silicates,  gelatine,  albumin),  A. 
Miiller4  (organic  colloids),  E.  Lacqueur  and  O.  Sackur5 
(casein),  Du  Pre  Denning6  (iron  hydroxide),  W.  B.  Hardy7 
(globulin,  etc.),  G.  Fano,  G.  Rossi,  O.  Scarpa,  J.  Simon8 
(albumin,  gum  arabic,  iron  hydroxide,  etc.),  S.  Axelrod9 
(rubber),  S.  J.  Levites10  (gelatine,  agar,  etc.),  W.  Flemming11 
(silicates),  Wo.  Pauli12  and  co workers  (albumin,  etc.),  Gokun13 
(gelatine),  W.  Frei14  (gelatine),-  V.  Albanese15  (albumin,  etc.), 
P.  Schidrowitz  and  Goldsbrough16  (rubber),  G.  Moruzzi17  (acid 

1  P.  von  Schroeder,  Z.  f.  physik.  Chem.,  45,  75  (1903). 

2  V.  Henri,  Lalou,  A.  Mayer,  Stodel,  Compt.  rend.  Soc.  de  Biologic,  55,  1668 

(1903)- 

3  H.  Garrett,  Diss.  Heidelberg,  1903;  Phil.  Mag.  (6),  6,  374  (1903). 

4  A.  Miiller,  Ber.  d.  Dtsch.  chem.  Ges.,  37,  u  (1903,  1904). 

6  E.  Laqueur  und  O.  Sackur,  Hofmeisters  Beitr.,  3,  193  (1903). 

6  Du  Pre  Denning,  Diss.  Heidelberg,  1904. 

7  W.  B.  Hardy,  Journ.  Physiol.,33,  251  (1905);  Proc.  Roy.  Soc.  B.,  79, 413  (1907). 

8  G.  Fano  und  G.  Rossi,  Arch,  di   Fisiol.,  i,  492,  609  (1904)  (rubber,  starch, 
serum);  G.  Rossi,  ibid.,  2,  500  (1905);  mit  O.  Scarpa,  ibid.,  2,  246  (1905)  (iron  hy- 
droxide); G.  Rossi,  ibid.,  2,  272,  599  (1905)  (albumin);  E.  Cavazzani,  ibid.,  2,  513 
(1905)  (milk);  G.  Rossi,  ibid.,  3,  171,  507  (1906)  (contains  a  review  of  the  litera- 
ture up  to  1906);  J.  Simon,  ibid.,  4,  594  (190?);  5,  394,  402,  470,  477,  479  (1908) 
(albumin  +  alcohol),  etc. 

9  S.  Axelrod,  Gummizeitung,  19,  1053  (1905);  20,  105  (1905);  23,  810  (1909). 
For  earlier  experiments  see  C.  O.  Weber,  Chemistry  of  India-rubber,  80,  London, 
1902. 

10  S.  J.  Levites,  Koll.-Zeitschr.,  2,  210  (1907). 


11  W.  Flemming,  Z.  f.  physik.  Chem.,  41,  407  (1907). 

/o.  Pauli  (in  part  with  H.  Handovsky,  K.  Schorr,  R.  Wagner,  Samec,  etc.) 
Hofmeisters  Beitr.,  n,  415  (1908);  Koll.-Zeitschr.,  3,  2  (1908);  Kolloidch.  Studien 


am.  Eiweiss,  Dresden,  1908;  Biochem.  Zeitschr.,  27,  296  (1910);  Sitz.  Ak.  Wiss.  Wien, 
17  Marz,  1910;  30  Juni,  1910;  Koll.-Zeitschr.,  7,  241  (1910). 

18  Gokun,  Koll.-Zeitschr.,  3,  84  (1908). 

14  W.  Frei,  Transvaal  Med.  Journ.,  Aug.,  1908. 

16  V.  Albanese,  Arch.  ital.  Biol.,  50,  387  (1909). 

16  P.  Schidrowitz  und  Goldsbrough,  Journ.  Soc.  Chem.  Ind.,  28,  3  (1909);  Koll.- 
Zeitschr.  4,  226  (1909). 

17  G.  Moruzzi,  Bioch.  Zeitschr.,  22,  232  (1909). 


154 


SPECIAL  COLLOID-CHEMISTRY 


albumin),  F.  Galdi1  (theory),  H.  Handovsky2  (albumin),  W. 
Biltz3  and  coworkers  (organic  dyes),  L.  Michaelis  and  B.  Mos- 
tynski4  (albumin),  F.  Bottazzi  and  C.  Victorow5  (soaps),  N. 
Sahlbom6  (iron  hydroxide  sol),  W.  E.  Ringer7  (acid  albumin), 
etc. 

Valuable  results  have  been  obtained,  of  which  only  the  more 
important  can  be  pointed  out  here. 

6.  Viscosity  Changes  in  Emulsoids  with  Time. — In  experi- 
ments on  the  viscosity  of  emulsoid  solutions  the  investigator 


Minutes   10 


20  30 

Time  — 


50 


60 


FIG.  23. — Increase  in  viscosity  of  emulsoids  with  time.     (According  to  the  experi- 
ments of  P.  wn  Schroeder,  S.  J.  Levites  and  W.  Biltz.} 

is  most  impressed  by  the  great  changes  in  viscosity  observable 
in  one  and  the  same  colloid  with  time.  To  be  sure  the  be- 
havior of  suspensoids  indicated  that  these  too  suffer  viscosity 
changes,  but  the  variations  occur  more  slowly.  " Spontaneous'' 

1  F.  Galdi,  II  Tammazi,  3,  Nr.  5;  Gior.  Ind.  Sc.  Med.  1909;  Rivist.  di  chem.  et 
micr.  clinic.,  9,  1909. 

2H.  Handovsky,  Bioch.  Zeitschr.,  25,510  (1910);  Koll.-Zeitschr.,  7,  183,267 
(1910). 

8  W.  Biltz  (with  A.  von  Vegesack,  Steiner,  etc.)  Z.  f.  physik.  Chem.,  73,  500 
(1910). 

4L.  Michaelis  and  B.  Mostynski,  Bioch.  Zeitschr.,  25,  401  (1910). 

•  F.  Bottazzi  and  C.  Victorow,  Rend.  R.  Ac.  Line.,  19,  659  (1910). 

6  N.  Sahlbom,  Koll.-chem.  Beih.,  2,  79  (1910). 

7  W.  E.  Ringer:  van  Bemmelen-Gedenkboek,  243  (1910);  this  volume  contains 
a  rich  literature. 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


155 


changes  in  viscosity  in  pure  suspense-ids  are  usually  to  be  reckoned 
in  days  (see  p.  151),  whereas  in  typical  emulsoids  they  often 
occur  from  minute  to  minute.  Another  very  interesting  dif- 
ference (which  may  possibly  serve  to  differentiate  suspension  from 
emulsion  colloids)  is  the  fact  that  the  viscosity  of  emulsoid  solu- 
tions usually  increases  with  time,  whereas  that  of  suspensoids 
decreases. 

Table  16  and  Fig.  23  give  a  picture  of  these  changes. 

TABLE  16. — INCREASE  IN  VISCOSITY  OF  EMULSOIDS  WITH  TIME 

(After  P.  von  Schroeder,  S.  J.  Levites,  and  W.  Biltz) 


Gelatine  solution 

(P.  von  Schroeder) 
viscosity 

Gelatine  solution 

(S.  J.  Levites) 
viscosity 

Benzopurpurin 

(W.   Biltz) 
0.4  per  cent.   (25°) 
viscosity 

Time 

At 

21.0° 

At 

24.8° 

At 
31.0° 

Time 

At 

25° 

Time 

Time  of 
outflow, 
seconds 

After    5  min.. 

1.83 

.65 

1.41 

After  15  min. 

2.  19    After    4  min. 

75-4 

After  10  min.  . 

2.10 

.69 

1.41 

After  30  min. 

2  .  39    After   7  min. 

75-8 

After  15  min.. 

2-45 

•74 

1.42 

After  45  min. 

2  .  59    After    9  min. 

77.0 

After  30  min.. 

4-13 

.80 

1.42 

After  60  min. 

2  .  80    After  13  min. 

81.2 

After  60  min.. 

13.76 

.90 

1.42 

After  75  min. 

3.00    After  3  1  min. 

106.0 

After  90  min. 

3.20    After  34  min. 

109.0 

After  ii  hr. 

3-40 

After  37  min. 

IIO.  2 

gelatinized. 

The  usual  way  in  which  viscosity  changes  with  time  is  prob- 
ably best  represented  by  the  S-shaped  curve  found  by  W.  Biltz 
and  A.  von  Vegesack  (I.e.)  in  their  experiments  on  benzopurpurin 
solutions.  In  other  words,  on  standing,  the  viscosity  of  an 
emulsoid  first  rises.  This  part  of  the  curve  corresponds  with 
the  first  portion  of  the  curves  obtained  for  gelatine  by  P.  von 
Schroeder  (I.e.).  Then  follows  an  almost  uniform  increase  in 
viscosity  as  shown  by  the  straight  middle  part  of  the  curve. 
This  straight  line  was  also  found  by  S.  J.  Levites  (I.e.).  Finally, 
there  follows  a  decrease  in  viscosity  which  is  represented  graphic- 
ally by  the  turn  of  the  curve  toward  the  abscissa.  This  late  be- 
havior is  found  not  only  in  the  case  of  benzopurpurin  but  also  in 
the  gelatine  curve  of  P.  von  Schroeder  at  24.8°  (shown  in  light 
type).  The  progressive  variations  in  the  viscosity  observed 
under  different  experimental  conditions  and  discussed  below  can 
all  be  interpreted  theoretically  in  the  terms  of  this  S  curve. 


156  SPECIAL  COLLOID-CHEMISTRY 

\ 

As  shown  by  the  experiments  of  P.  von  Schroeder  (I.e.)  on 
gelatine,  the  changes  in  viscosity  with  time  decrease  absolutely 
as  well  as  relatively  with  rise  in  temperature.  It  is  also  an 
important  fact  that  the  changes  with  time  are  the  more  distinct 
the  more  concentrated  the  solution.  Under  certain  conditions 
dilute  emulsoids  behave  like  molecular  solutions,  that  is,  they  show 
a  constant  or  perhaps  a  decreasing  viscosity  (see  below,  p.  157). 
Thus,  W.  Biltz  and  H.  Steiner  (I.e.)  could  observe  well  marked 
changes  in  state  in  night-blue  solutions  at  50°  only  if  they  con- 
tained more  than  3  per  cent,  of  the  dye.  The  effect  of  the  addi- 
tion of  various  substances,  especially  salts,  upon  the  time-varia- 
tions in  viscosity  was  studied  by  Gokun  (I.e.),  who  found  that 
when  small  amounts  (of  ammonium  nitrate)  were  added,  the  vis- 
cosity of  gelatine  solutions  increased  more  quickly  than  without 
such,  while  if  larger  amounts  were  added  (say  0.32  to  1.4  normal 
NEU.NOa)  the  viscosity  remained  almost  constant.  In  very  high 
concentrations  (5.6  to  6.4normalNH4.NO3)  the  viscosity  decreased 
with  time  as  in  the  case  of  suspensoids.  The  latter  is  doubtless 
due  to  the  fact  that  in  such  high  concentrations  "precipitation 
effects,"  in  other  words,  coagulatory  effects  begin  to  manifest 
themselves.  The  following  table  gives  some  illustrative  figures 
which  are,  however,  not  very  exact. 


TABLE  17. — EFFECT  OF  THE  ADDITION  OF  NH4NO3  ON  THE  CHANGE  IN  VISCOSITY 

OF  A  GELATINE  SOLUTION  (0.28  PER  CENT.)  WITH  TIME 

(After  Gokun) 

Concentration  of  the  added  salt  in  normality 


o 

0.175 

0.35 

0.7 

I.  OS 

1-4 

2.  I 

2.8 

3.5 

4.2 

S.6 

After    44  hours  

1.36 

1.77 

.15 

.09 

1.07 

I.  OS 

I.  OS 

I.  OS 

1.07 

1.  13 

1.30 

After    44  hours  

1.42 

1.  80 

•  70 

.41 

1.35 

I.  12 

1.05 

I.  OS 

1.06 

1.08 

i.  18 

After    67  hours  

1.62 

i.os(?) 

•  77 

•  54 

1.32 

I.  IS 

1.07 

1.  01 

i.  06 

1.08 

1.  18 

After    91  hours  

1.77 

1.90 

.82 

•  S0(?) 

1.32 

1.  17 

1.07 

1.05 

i.  08 

1.08 

i.  IS 

After  115  hours  

2.  02 

2.06 

.81 

.77 

1.37 

1.  17 

1.05 

1.  01 

1.02 

1.07 

1.  14 

For  the  influence  of  non-electrolytes  the  reader  is  referred  to 
the  experiments  of  J.  Simon  (I.e.,  1907-08)  on  the  effect  of  alcohol 
on  the  increase  of  viscosity  with  time  in  albumin  solutions.  This 
author  found  that  the  viscosity  increased  as  he  added  more  alcohol. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


157 


Acetone  had  a  similar  influence  while  the  higher  alcohols  were  less 
effective.     Fig.  24  gives  a  picture  of  the  results. 

A  case  differing  from  those  hitherto  mentioned  in  that  it 
concerns  an  emulsoid  which  spontaneously  grows  less  viscid  with 
time  has  been  described  by  H.  W.  Woudstra1  in  his  work  on  the 
toluol  sols  of  rubber.  As  he  has  made  only  a  preliminary  statement 
we  cannot  be  sure  that  this  case  is  a  true  exception.  Woudstra 
found  his  carbon  tetrachloride  sols  to  become  cloudy  with  time.2 


38% 


35% 


20% 


5  10         12    Hours.  + 

Time  > 

FIG.  24. — Effect  of  time  upon  the  viscosity  of  serum  albumin  to  which  alcohol  has 
been  added.     (According  to  /.  Simon.} 

It  seems  possible,  therefore,  that  under  the  experimental  conditions 
chosen  by  this  investigator  his  solutions  coagulated,  in  which  case 
their  behavior  would  naturally  be  irregular.  Their  method  of 
preparation  (swelling,  trituration,  nitration  through  glass  wool) 
may  also  have  influenced  his  findings.  His  solutions  may  have 
contained  "  undissolved "  particles  which  at  first  caused  a 
high  viscosity,  but  which,  later,  after  their  " solution,"  led  to 

1  H.  W.  Woudstra:  Kolloid-Zeitschrift,  5,  33  (1909). 

2  Dr.  Brauer  of  Leipzig  has  also  observed  that  filtered  solutions  of  purified 
rubber  which  are  originally  entirely  clear  show  flocculi  after  standing  some  weeks. 
Since  then  I  have  been  able  to  make  analogous  observations  on  benzol  rubber  sols. 


158  SPECIAL  COLLOID-CHEMISTRY 

decrease  in  the  viscosity.     Further  experiments  are  needed  on 
this  point. 

In  accord  with  Woudstra's  observations  are  those  of  K.  Schorr 
and  H.  Handovsky  (I.e.,  1910)  who  found  that  albumin  solutions 
first  show  a  gradual  increase  in  viscosity  but  later  a  slow  de- 
crease on  the  addition  of  alkali.  Chemical  changes  (hydrolytic 
cleavage,  etc.)  which  produce  secondarily  a  decrease  in  viscosity, 
somewhat  analogous  to  the  hydrolytic  action  of  ferments,  are 
undoubtedly  active  here  (see  p.  160). 

7.  Effect  of  Mechanical  Treatment  on  Viscosity  of  Emulsoids. 
— It  is  a  remarkable  fact  that  the  viscosity  of  emulsoids  is  affected 
by  mechanical  treatment.     If  they  are  shaken  for  a  period  or  simply 
pressed  several  times  through  a  capillary,  as  in  a  viscosimeter, 
their  viscosity  decreases.     Such  phenomena  have  been  observed 
by  Gokun  (I.e.)  and  W.  Biltz  (I.e.).     They  show  that  even  in  such 
apparently  perfect  liquids  there  is  present  a  kind  of  "structure" 
which  is  destroyed  by  mechanical   treatment.     This   structure 
seems  closely  allied  with  the  oft-mentioned  liquid  membranes  of 
the  dispersion    medium  which   surround  the  disperse   particles 
and  which  we  used  above  to  explain  the  first  maximum  viscosity 
observed  in  suspensoids  (seep.  146).     One  may  imagine  that  in 
higher  concentrations  these  membranes  unite,  somewhat  as  repre- 
sented in  Fig.  14  on  p.  87,  and  that  mechanical  treatment  pulls 
the  individual  envelopes  apart  again.     In  favor  of  this  view  is 
the  fact  that,  according  to  W.  Biltz  and  H.  Steiner,  this  phenome- 
non is  particularly  marked  in  concentrated  solutions ;  and  that  the 
viscosities  of  solutions  of  different  ages  may  be  reduced  to  the  same 
value   by  sufficient  shaking  (see  Table  iS).1     The  matter  will 
be  taken  up  more  fully  later. 

TABLE  18. — INFLUENCE  OF  SHAKING  ON  THE  VISCOSITY  OF  A  2.7  PER  CENT. 

SOLUTION  OF  NIGHT-BLUE 
(According  to  W.  Biltz  and  H.  Steiner) 

Without  shaking  After  shaking 

a  b 

151.5  118.2  117.0 

143.4  118.0  117.0 

139-9  118.4  II7-4 

8.  Influence  of  "Inoculation"  on  Internal  Friction  of  Emul- 
soids.— A  remarkable  phenomenon  has  been  observed  by  H.  Gar- 

1  In  passing  it  may  be  mentioned  that  H.  Zangger  observed  ordinary  milk  to 
show  this  behavior. 


MECHANICAL  PROPERTIES   OF   COLLOID    SYSTEMS 


159 


rett  (I.e.)  in  solutions  of  gelatine  and  by  W.  Biltz  and  H.  Steiner 
(I.e.)  in  solutions  of  night-blue.  They  found  the  spontaneous  in- 
creases in  viscosity  which  such  show  to  be  markedly  accelerated 
through  the  addition  of  small  quantities  of  aged  or  gelatinized 
solutions. 

TABLE  19. — INFLUENCE  OF  INOCULATION  ON  THE  INTERNAL  FRICTION  OF  A 

SOLUTION  OF  TECHNICAL  NIGHT-BLUE  AT  25° 

(According  to  Biltz  and  Steiner) 

Time  of  outflow 


Without  inoculation 

After  inoculation 

Per  cent. 

At  once 

After 
i  day 

After 
6  days 

At  once 

After 
^hr. 

After 
i  hr. 

After 
2hr. 

After 
i  day 

0.90 

1-35 
i.  80 
2.25 

77-  2" 
79-3 
77-6 

85.2 

78.5" 
82.0 
85.6 
Qi-3 

78.5" 
81.6 

79-2" 
82.2 
83-2 
88.9 

78.8" 
82.0 
85.9 

85.2 
88.9 

86.1 
91.6 

85.6 
103-3 

IO2  .6 

As  the  table  shows,  this  behavior  is  best  observed  only  in 
colloid  solutions  of  high  concentration. 

It  should  be  emphasized,  as  P.  von  Schroeder  (I.e.)  has  shown, 
that  this  phenomenon  depends  upon  a  chemical  change  in  the 
gelatine,  probably  upon  its  hydrolytic  cleavage.  This  is  proved 
not  only  by  the  fact  that  the  decrease  in  viscosity,  with  pro- 
longed heating,  is  irreversible,  but  also  by  the  fact  that  it  follows 
the  laws  of  chemical  mass  action.  Furthermore,  after  prolonged 
heating,  precipitates  appear  in  the  solution,  which  I  hold  to  be  the 
products  of  this  chemical  reaction.1  Analogous  considerations 
apply  to  the  changes  in  viscosity  which  silicic  acid,  etc.,  show  when 
heated  or  otherwise  treated  (W.  Fleming,  I.e.).  This  view  is 
also  supported  by  the  fact  that  long  heating  decreases  the  vis- 
cosity of  many  emulsoids,  though  by  no  means  all.  W.  Biltz 
and  H.  Steiner  (I.e.),  for  example,  found  that  emulsions  of  night- 
blue  do  not  alter  their  viscosity  even  after  heating  7  hours. 

What  has  been  said  under  headings  3  to  8  must  always  be 
borne  in  mind  when  making  viscosity  determinations  on  emulsoids. 
For  this  reason  the  discussion  entered  into  there  needed  to  pre- 
cede a  consideration  of  the  relations  between  internal  friction, 
concentration,  temperature,  etc. 

9.  Influence  of  Thermal  History  on  Viscosity  of  Emulsoids. — 
When  such  typical  emulsoids  as  gelatine,  agar-agar,  etc.,  are  sub- 
1  Wo.  Ostwald,  Pfliiger's  Arch.,  109,  277  (1905). 


i6o 


SPECIAL  COLLOID-CHEMISTRY 


jected  to  the  influence  of  heat  their  viscosity  is  affected  in  the 
same  way  and  as  markedly  as  when  they  are  treated  mechanically. 
Prolonged  heating  decreases  the  internal  friction  of  these  solu- 
tions. By  prolonged  boiling,  it  is  possible  to  so  change  a  solu- 
tion of  gelatine  or  glue  that  it  will  no  longer  solidify  when  cooled. 
When  alcohol  is  added  to  a  gelatine  solution  thus  altered  by  pro- 
longed boiling,  a  yellow  precipitate  is  thrown  down,  which  is 
easily  soluble  in  water.  A  precipitate  similarly  produced  in 
normal  gelatine  only  "  swells  "  when  thrown  into  cold  water.  This 
was  observed  as  early  as  1867  by  Moritz  Traube.1  Traube  called 
the  modification  which  would  no  longer  gelatinize,  /3  gelatine  or 
]8  glue  in  contrast  to  the  normal,  gelatinizing  a  form.  Table 
20  and  Fig.  25  copied  from  P.  yon  Schroeder  (I.e.)  illustrate  what 
has  been  said.  S.  J.  Levites2  has  made  further  experiments  on 
purified  gelatine  (gluten),  agar-agar  and  on  the  sodium  salt  of 
thymonucleic  acid  with  entirely  analogous  results. 

TABLE  20. — INFLUENCE  OF  HEATING  ON  THE  VISCOSITY  OF  GELATINE  SOLUTIONS 
(According  to  P.  von  Schroeder) 

Internal  friction  of  gelatine 


about  100° 
i  per  cent.                       2  per  cent. 

3  per  cent. 

0.5                                         .29                                  1.75 

i.o                                        .23                                1.55 

1.5                                                     .20                                           1.49 

.... 

2.0                                                     .17                                           1.47 

1.76 

2-5                                                     -IS 

3-0                                      -J4                               i-37 

1.68 

3-5                                      -!3 



4-0                                      .13                               1-32 

1.56 

4-5                                     -11 

5-5                                  I-" 



6.0                                   1.28 

1.50 

7.0                                   i  .26 

•47 

8.0                                  ....                              1.25 

•47 

9.0 

•44 

10.  o                                  ....                              1.24 

•42 

12.0                                                 1.23 

.40 

I4.O                                                 ....                                           1.22 

•39 

l6.O                                                 ....                                            1.22 

•39 

1  M.  Traube,  Reichert  and  Du  Bois  Reymond's  Arch.,  87  (1867). 

2  S.  J.  Levites,  Koll.-Zeitschr.,  2,  239  (1907). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


161 


io.  Influence  of  Concentration  on  Internal  Friction  of  Emul- 
soids. — The   influence   of   concentration   upon   the   viscosity   of 


FIG.  25.- 


5  10 

Number  of  hours  heated 

-Effect  of  prolonged  heating  on  the  viscosity  of  a  gelatine  solution, 
(According  to  P.  von  Schroeder.) 


3,00 


Z50 


2,00 


V50 


1,00 


Concentration 


FIG.  26. — Influence  of  concentration  on  viscosity  of  gelatine  solutions  at  35°. 
(According  to  S.  J.  Levites.) 

emulsoids  simulates  its  effect  upon  suspensoids.     This  is  clearly 
evident  on  comparing  Figs.  26  and  22,  in  doing  which  it  is  well  to 


ii 


162 


SPECIAL  COLLOID-CHEMISTRY 


limit  oneself  to  comparison  involving  the  same  temperatures. 
Examples  of  the  effects  of  concentration  are  given  in  the  follow- 
ing Tables  21  and  22. 

TABLE  21. — INFLUENCE  OF  CONCENTRATION  ON  VISCOSITY  OF  GELATINE 

SOLUTIONS 
(According  to  S.  J.  Levites) 


a  Gelatine,  at  35° 

«  Gelatine,  at  35° 

Per  cent.                             Viscosity 

Per  cent. 

Viscosity 

0.25 

1.  10 

0-5 

.186 

0-5 

I  .22 

I  .O 

.262 

0-75 

1.32 

i.  5 

•332 

1  .0 

1.46 

2.0 

•432 

i.  5 

1-75 

3-0 

.603 

2.0 

2.05 

4.0 

.856 

3-o 

2.96 

Emphasis  should  be  laid  on  the  fact  that  the  above  measure- 
ments refer  either  to  low  colloid  concentrations  or  were  obtained  at 
higher  temperatures.  As  every  one  who  has  experimented  with 
gelatine  or  agar-agar  well  knows,  there  is,  for  every  typical  emulsoid, 
an  optimum  concentration  and  an  optimum  temperature  at  which 
the  solution  gelatinizes.  Thus  solutions  of  night-blue  above  1.575 
per  cent,  are  so  thick  at  o°  that  they  no  longer  flow  through  a 
viscosimeter.  We  wish  here  merely  to  point  out  that  the  influ- 
ence of  concentration  on  viscosity  in  typical  emulsoids  is  very 
great.  Thus  the  viscosity  of  an  agar-agar  solution  (at  room 
temperature)  varies  within  the  first  2  per  cent,  from  that  of  pure 
water  to  that  of  a  solid.  If  one  compares  molar  instead  of  per- 
centage concentrations,  the  great  absolute  increases  in  the  value  as 
well  as  the  abruptness  of  the  viscosity  changes  appear  still  more 
striking. 

The  effect  of  temperature  on  the  concentration  influence  is  such 
that  decreasing  the  temperature  makes  the  ascent  of  the  curve 
steeper,  while  increasing  the  temperature  flattens  it  (see  Fig.  27). 
This  behavior  is  analogous  to  that  observed  in  molecular  dis- 
persoids  and  probably  to  that  observed  in  suspensoids.  Added 
substances  like  salts  increase  or  decrease  the  slope  of  the  curve  as 
do  temperature  changes.  Purification  of  the  technical  night-blue 


MECHANICAL  PROPERTIES   OF    COLLOID    SYSTEMS  163 

TABLE  22. — INFLUENCE  OF  CONCENTRATION  ON  VISCOSITY  OF  NIGHT-BLUE 

SOLUTIONS 
(According  to  W.  Biltz  and  H.  Steiner) 


Technical  night-blue 

Purified  night-blue 

At  50° 

At  25° 

Ato° 

At  25° 

Concen- 
tration, 
'  per  cent. 

: 

Internal 
friction 

1 
!    Concen- 
tration, 
per  cent. 

Internal 
i     friction 
after  6  days 

Concen- 
tration, 
per  cent. 

Internal 
friction 

Concen- 
tration, 
per  cent. 

Internal 
friction 

0.225 

.007 

0.025 

0.985 

0.225 

1.009 

0.25 

1.008 

0-45 

.019 

0.045 

0.990 

0.45 

1.026 

0.50 

1.027 

0.675 

.027 

0.090 

0.994 

0-675 

1.042 

0-75              -058 

0.90 

.041 

0.145 

0.997 

0.90 

1.068 

I  .  00                .  068 

I.I25 

.054 

0.180 

0.996            I.I25 

I  .IOI 

1.25 

.091 

i-3S 

.071 

0.225 

1.  006 

1-35 

1.132 

1.50 

.106 

i.  575 

.090 

0.270 

.006 

1-575 

1.  176 

1-75 

•145 

i.  80 

.097 

0.315 

.006           I.  80 

1.180 

2.OO                .171 

2.025 

•125 

0.360 

.008 

2.25 

.221 

2.25 

.142 

0.405 

.014 

2.50 

.263 

2-475 

•157 

0.450 

.019 

2.75 

-334 

2.70 

.178 

0.495 

.O2O 

3-oo 

.403 

3.15 

.240 

0.540 

•033 

3.60 

.298 

0.6075 

.037 

4-05 

•393 

0.675 

.042 

4-50 

•455 

0.7875 

.054 

0.900 

.022 

1.0125 

.065 

1.125 

.080 

1.237 

.110 

1-35 

.105 

1-575 

•139 

i.  80 

.182 

2.025 

.272 

2.25 

•390 

2-475 

.480 

2.70             .525 

(which  is  ordinarily  contaminated  by  about  43  per  cent,  sodium 
sulphate)  decreases  the  slope  of  the  curve,  that  is,  has  the  same 
effect  as  raising  the  temperature.  It  is  not  impossible,  however, 
that  the  addition  of  other  salts,  such  as  the  chlorides,  nitrates,  etc., 
might  have  an  opposite  effect.  Chemical  changes  in  the  colloid 
itself  also  change  the  character  of  the  concentration  curve,  as  is 
evident  in  the  tables  and  curves  referring  to  a  and  0  gelatine. 
A  mathematical  definition  of  the  influence  of  the  concentration 


164 


SPECIAL  COLLOID-CHEMISTRY 


on  the  viscosity  of  emulsoids,  in  other  words,  an  equation  adequate 
for  the  whole  range  of  concentrations  has  not  yet  been  formulated. 
But  this  is  also  true  of  molecular  dispersoids  [see  S.  J.  Levites  (I.e.), 
where  references  to  the  literature  may  be  found].  Yet  the  regu- 
larity of  the  Biltz  curves  (Fig.  27)  indicates  that  a  general,  even 
though  empirical,  equation  may  be  worked  out.  I  have  purposely 
inserted  the  numerous  tables  in  the  text  to  excite  interest  in  this 
direction. 


7-o 


50° 


Cone  enf-ration 
I  i 


/  2  3  V  5% 

FIG.  27. — Influence  of  concentration  on  the  viscosity  of  night-blue  solutions. 
(According  to  W.  Biltz  and  H.  Steiner.}  The  curve  marked  "  G"  shows  the  behavior 
of  the  purified  night-blue. 

ii.  Influence  of  Temperature  on  Viscosity  of  Emulsoids. — 
Besides  the  influence  of  concentration  on  the  viscosity  of  emulsoids, 
described  in  the  previous  paragraphs,  there  exists  also  a  relation 
between  temperature  and  viscosity  which  is  observed  when  all  other 
factors  are  kept  constant.  But  systematic  investigations  of  this 
type  over  a  larger  temperature  range  have  not  as  yet  been  made. 
For  reasons  already  given,  only  dilute  emulsoids  can  be  used  for 
such  study.  Some  approximate  determinations  of  the  average 
temperature  coefficients  of  the  viscosity  of  emulsoids  are,  however, 
at  hand.  Thus  the  internal  friction  of  pure  water  changes  about 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  165 

18  per  cent,  between  21°  and  3i°C.  In  contrast  to  this,  the 
viscosity  of  a  3  per  cent,  gelatine  solution,  within  the  same  tem- 
perature range,  changes  from  1.42  to  13.76,  in  other  words,  almost 
1000  per  cent.  (P.  von  Schroeder,  I.e.).  According  to  Biltz  and 
Steiner  (I.e.),  the  absolute  viscosity  of  a  1.8  per  cent,  solution  of 
night-blue  rises  from  10.5  to  32  between  25°  and  o°,  in  other 
words,  triples,  while  the  viscosity  of  water  merely  doubles 
under  the  same  conditions.  With  higher  concentrations  the 
changes  in  viscosity  within  very  narrow  ranges  of  temperature 
become  extraordinary,  for  the  existence  of  gelatination  and  melting 
points  means  nothing  else  but  that,  within  a  temperature  change 
of  a  degree  or  less,  the  viscosity  of  such  systems  changes  from 
that  of  a  fluid  to  that  of  a  solid. 

12.  Influence  of  Added  Substances  on  Viscosity  of  Emul- 
soids. — The  influence  of  added  substances  on  viscosity,  when  all 
other  external  factors  have  been  kept  constant,  has  also  been 
thoroughly  investigated.  Of  the  mass  of  facts  available  in  this 
field  we  shall  mention  only  a  few.  For  details  the  original  papers 
should  be  consulted. 

So  far  as  the  important  effect  of  salts  upon  emulsoids  is  con- 
cerned, the  accuracy  of  most  of  the  earlier  measurements  is  vitiated 
because  impure  preparations,  contaminated  with  electrolytes,  were 
used.  Only  recently  have  Wo.  Pauli  and  his  co workers  (/.c.), 
in  a  careful  and  searching  series  of  investigations,  shown  what 
minute  amounts  of  electrolytes  suffice  to  cause  substantial  changes 
in  the  viscosity  of  organic  emulsoids.  Nevertheless  older  experi- 
ments with  commercial  preparations  and  those  purified  by  ordinary 
laboratory  methods  are  not  valueless,  for  such  colloids  are  used  in 
many  of  the  arts  and  for  some  scientific  purposes. 

We  must  distinguish  between  the  effects  of  salts  on  the  vis- 
cosity of  emulsoids  which  with  time  are  either  stable  or  un- 
stable. When  of  the  latter  class,  as  with  gelatine,  a  distinction 
must  be  made  between  the  initial  value  of  the  viscosity  as  ob- 
served immediately  after  the  addition  of  a  salt  and  the  final  value 
which  is  approached  only  asymptotically.  According  to  the 
experiments  of  P.  von  Schroeder  (l.c.),  S.  J.  Levites  (I.e.)  and 
Gokun  (I.e.),  the  first  of  these  values  follows  the  general  rule  of 
mixtures:  salts  which  raise  the  internal  friction  of  water  affect 
colloid  solutions  similarly,  and  vice  versa.  The  final  value  ex- 


i66 


SPECIAL  COLLOID-CHEMISTRY 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS 


i67 


hibited  by  gelatine  solutions  after  the  addition  of  salts  is  very 
different  from  this  first.  In  Table  23  and  Fig.  28,  taken  from 
P.  von  Schroeder  (I.e.),  are  collected  a  series  of  such  viscosity 

TABLE  23. — A.  INFLUENCE  or  SALTS  ON  INTERNAL  FRICTION  OF  GELATINE 
(After  standing  i  hour) 


Salt 

Concentration 

Salt 

Concentration 

H  norm. 

^  norm. 

}4  norm. 

M 

norm. 

y* 

norm. 

% 
norm 

i 
norm. 

Pure  gelat.  . 

l.78 

I 

.73 
.72 

.21 

1.78 

Pure  gelat... 

1.88 

1.70 

1-83 

I.7I 

Na2SO4  

2.  II 
I.Q7 

i.  95 

2 
2 

9.41 
3-32 

NaCl...     . 

l.76 
1.  80 
1-73 

I.7I 
1.67 
1  .69 

1.74 
1.  60 
1.  60 

1-59 
LSI 
I-SI 

K2SO4 

KC1 

(NH4)2S04 

NH4C1  .... 

Pure  gelat.  . 

1.68 

1.68 

1.68 

Pure  gelat  .  . 

1.65 

1.68 

l.76 

1.70 

Concentration 

NaNO3.     .  . 

1.63 
1.65 
1.61 

i.  57 
1-53 
1.52 

1.56 
1.52 
1.49 

KNO3 

1.48 
i.45 

H« 
norm. 

M 

norm. 

X 

norm. 

M 

norm. 

NH4N03.... 

LiCl  

1-73 
i.76 
1.92 

2.12 

I.78 
2.0C 

2.15 

2.42 

1.66 
1.88 

MgCl2 

Li2S04  
MgS04  

I.8S 
I.QO 

B.  DIFFERENCES  BETWEEN  INTERNAL  FRICTION  OF  SALT-GELATINE  AND  PURE 
i  PER  CENT.  GELATINE 


Na 

K 

NH4 

Mg 

Li 

SO  A  \4  R  norm 

_|_Q       Tly 

SO4  %  norm  

-J-O    77 

-}-o  oo 

•4~O    17 

-\-Q     A  A 

-i-n    24. 

SO4  J^  norm.   . 

-j-T    OI 

-4-n  4.8 

-\~O    QA. 

-|_O      A*J 

SO4  ]/2  norm 

-4-7     67 

-l-i  6  A 

Cl  %  norm  

—  o  i  <\ 

-J-O    TO 

-4~o  o1* 

Cl  ^  norm. 

—  O    12 

—  o  08 

—  o  02 

Cl  ^2  norm  

-j-o  01 

—  o  03 

—  O    27 

-4-o    72 

-j-Q     2O 

Cl  i  norm  

—  O    OQ 

—  o  20 

—  o  20 

NO  3  %  norm. 

—  O    12 

o  oo 

NOs  24  norm  

—  O    O2 

—  o  i<? 

—  o  16 

NOs  ^i>  norm  

—  O    1  1 

—  O    24. 

—  O    27 

NO  3  i  norm. 

—  o  20 

—  O    72 

—  O    2C 

v.  ^ 

The  plus  sign  means  that  the  internal  friction  of  the  salt-gelatine  is  greater  than 
that  of  the  pure  gelatine,  and  the  minus  sign  the  reverse. 


1  68  SPECIAL  COLLOID-CHEMISTRY 

values  in  gelatine  solutions  which  have  stood  for  an  hour.  Table 
23,  B,  details  the  difference  in  viscosity  between  salt-gelatine  and 
pure  gelatine.  If  the  difference  is  positive  it  means  that  the 
viscosity  of  the  gelatine  has  been  increased  by  adding  the  salt, 
while  if  it  is  negative  it  means  that  the  viscosity  has  been  reduced 
to  below  that  of  pure  gelatine. 

It  appears  that  sulphates  in  all  concentrations  increase  the 
internal  friction  of  gelatine,  while  chlorides  and  nitrates  decrease 
it,  with  the  exception  of  MgCl2  and  LiCl  in  higher  concentrations. 
The  exact  concentration  of  the  salt,  however,  plays  an  important 
part,  especially  in  the  chlorides  which  in  medium  concentrations 
(about  J£  normal)  show  a  maximum  of  viscosity  which  sometimes 
exceeds  that  of  pure  gelatine.  Further  details  may  be  found  in 
the  tables  and  curves.1 

If  the  anions  of  the  added  salts  are  arranged  according  to 
their  effect  we  obtain  the  series: 

SO4>C1>N03 

In  the  case  of  the  kations  variations  occur  with  different 
concentrations.  If  we  choose  the  values  found  for  J^  normal 
solutions  we  find  that  the  sulphates  and  the  chlorides  arrange 
themselves  as  follows: 

Mg>Na>Li>NH4>K 

Ample  opportunity  will  be  found  later,  to  return  to  these 
"ionic  series,"  which  in  honor  of  the  investigator  who  discovered 
them  are  now  known  as  the  Hofmeister  series.  There  we  shall 
also  find  that  the  complicated  influence  of  the  concentration  of  a 
salt  is  not  an  accidental  or  an  exceptional  one,  but  an  expression 
of  general  characteristics  of  the  relation  between  any  salt  and  a 
change  in  the  state  of  the  colloid  system. 

P.  von  Schroeder  (I.e.)  has  investigated  the  important  in- 
fluence of  acids  and  alkalies  on  the  viscosity  of  gelatine  solutions. 
His  findings  are  detailed  in  Table  24  and  Fig.  29.  The  influence 
of  concentration  is  again  complex,  for  at  certain  low  concentrations 
(3^56  normal  for  HC1  and  J^28  normal  for  NaOH)  a  maximum 


1  It  should  again  be  emphasized  that  pure  gelatine  would,  perhaps,  show  totally 
different  results. 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  1 69 

viscosity  is  attained,  while  at  concentrations  above  ^2  normal  a 
viscosity  below  that  of  pure  gelatine  is  observed. 


Pure 


ConcenTraTion 


1/2  normal 


FIG.  29. — Effect  of  HC1  and  NaOH  upon  the  viscosity  of  gelatine.    (According  to 

P.  von  Schroeder.') 

TABLE  24. — INFLUENCE  OP  HC1  AND  OF  NaOH  ON  VISCOSITY  OF  GELATINE 
(According  to  P.  von  Schroeder) 


HCl 


Concentration 

Viscosity 

Concentration 

Viscosity 

O 

.40 

O 

.40 

/^12  norm. 

•55 

x^jl2  norm. 

•52 

^56 

.76 

M56 

.60 

H28 

.68 

H28 

•79 

/^4 

•58 

x^4 

.62 

Yzi 

.42 

/^2 

.38 

He 

•25 

He 

•25 

M 

•17 

« 

.10 

H 

I.  12 

M 

.10 

NaOH 


Similar  effects  of  concentration,  more  especially  of  the  alka- 
lies, on  the  viscosity  of  soap  solutions  have  been  observed  by  F. 
Bottazzi  and  C.  Victorow  (I.e.). 

13.  Effect  of  Added  Substances  on  Internal  Friction  of  Emul- 
soids;  Behavior  of  Protein  Solutions. — Through  the  work  of  E. 
Laquer  and  O.  Sackur  (I.e.),  W.  B.  Hardy  (I.e.)  and  others,  and 
especially  through  that  of  Wo.  Pauli  and  his  coworkers  (I.e.),  we 


170 


SPECIAL  COLLOID-CHEMISTRY 


have  become  better  acquainted  with  the  behavior  of  various  pro- 
tein solutions  such  as  those  of  serum  albumin,  egg  albumin, 
globulin  and  casein  in  the  matter  of  their  viscosity  when  subjected 
to  the  effects  of  added  chemical  substances.  These  solutions  be« 
long  to  the  emulsoids.  Time  alone  changes  their  internal  friction, 
yet  these  changes  take  place  so  rapidly  that  the  final  viscosity 
value  is  reached  within  a  few  minutes.  Because  of  this  and  because 


1250: 


Monochloracetic 
Acid 


0,0050,01       0,02      O03        0.04       OjOSn 

FIG.  30. — Influence  of  acids  upon  the  viscosity  of  serum  albumin. 
Wo.  Pauli  and  H.  Handovsky.)     17  means  viscosity. 


(According  to 


the  proteins  can  be  isolated  and  better  purified  than  gelatine,  for 
example,  they  adapt  themselves  especially  well  to  a  study  of  this 
important  problem. 

The  most  striking  fact  that  the  study  of  the  influence  of  elec- 
trolytes on  the  viscosity  of  purified  proteins  has  brought  out  is 
the  enormous  change  in  viscosity  which  is  produced  by  traces  of 
electrolytes.  This  is  especially  true  of  acids1  and  alkalies  which 

1  Regarding  the  effect  of  acids,  more  especially  of  acetic  acid  on  protein,  see  the 
paper  of  L.  Zoja,  Koll.-Zeitschr.,  3,  249  (1908). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS 


171 


TABLE  25. — A.  INFLUENCE  OF  ACIDS  ON  VISCOSITY  OF  SERUM  ALBUMIN 
(According  to  Wo.  Pauli  and  H.  Handovsky)1 


Concentration 


Internal  friction 


HC1 


Citric 
acid 


Oxalic 
acid 


o.oo  norm. 

0.005 

o.oi 

0.015 

0.017 

0.02 
0.03 
O.O4 
0.05 


I . 0409   ! 
1.0832   I 
I. 1660 
1.2432 
1.2432 
1.2323 
1.1647 
I.I3S6 
I . 1206 


I . 0409         I . 0409 


Sulphuric  [  Trichloracetic 


acid 


acid 


1.0409 


1.0442         I. 0688 


1.0613 


I .0661 
I . 1002 
I  .1112 
I . 1408 


I.I337  I.06I3 

1.1634  1.0604 

1.1852  !  1.0638 

1.1700  i  1.0656 


1.0409 
1.0511 
1.0725 


1.0594 
1.0525 
1.0564 
i . 0603 


Acetic 
acid 


1.0409 


1.0456 


1.0518 
1.0658 
1.0751 
I . 0906 


B.  INFLUENCE  OF  BASES  ON  VISCOSITY  OF  SERUM  ALBUMIN 
(According  to  Wo.  Pauli  and  H.  Handovsky) 


Base 


Concentration 


Sodium  hydroxide I         o .  01  norm. 

0.02 
0.03 

Ammonia o.oi  norm. 

0.03 
0.05 

Triethylamine o.oi 

0.03 
0.05 

Ethylamine o.oi 

0.03 
0.05 

Methylamine o.oi 

0.05 

Diethylamine o .  01 

0.03 
0.05 

Piperidine o.oi 

0.03 
0.05 

Tetraethylammoniurn 

hydroxide o .  01 

0.03 
0.05 


Friction  increase  in 
per  cent. 


78 

ISI 

195 

19 

23 

28 

20 

28 

33 

37 

65 

83 

40 

76 

52 

103 

146 

53 
109 


116 

221 
230 


Concentration, 
OH'.io-* 


96o 
1900 
2805 

49 

82 

108 

85 
148 
196 
214 
390 
465 
204 
442 
308 

564 
800 

334 
627 
825 

922 
2718 
4490 


1  See  H.  Handovsky,  Koll.-Zeitschr.,  7,  268  (1910). 


172 


SPECIAL  COLLOID-CHEMISTRY 


show  a  behavior  entirely  analogous  to  that  discussed  in  con- 
nection with  gelatine  on  p.  169.  Thus  Wo.  Pauli  and  H.  Han- 
do  vsky  (I.e.)  found  that  the  addition  of  0.015  normal  HC1  suf- 
fices to  raise  the  viscosity  of  a  serum  albumin  solution  from 
1.0623  to  1.2937,  in  other  words,  more  than  20  per  cent.  With 
alkalies,  a  concentration  of  Hoo  normal  tetraethylammonium 
hydroxide  is  enough  to  increase  the  viscosity  230  per  cent.  Table 
25  and  Figs.  30  and  31  may  serve  to  illustrate  these  facts. 


O.OJn 

Concentration  of  the  Base 

FIG.  31. — Influence  of  bases  upon  the  viscosity  of  serum  albumin.     (According  to 
Wo.  Pauli  and  H.  Handovsky.} 

So  far  as  the  effect  of  salts  is  concerned,  it  is  found  that  this 
is  different  depending  upon  whether  neutral,  acid  or  alkaline 
albumin  is  used  (Wo.  Pauli).  The  relations  are  complicated 
especially  when  the  effects  of  different  concentrations  of  acids  and 
alkalies  as  well  as  of  salts  are  considered.  It  remains  for  future 
investigators  to  give  us  a  clear  and  comprehensive  presentation 
of  this  subject. 

The  following  features  deserve  emphasis :  Neutral  salts  always 
lower  the  viscosity  of  neutral  protein  (Wo.  Pauli).  This  be- 


MECHANICAL   PROPERTIES   OF   COLLOID    SYSTEMS  173 

havior  is  analogous  to  the  effects  of  salts  on  the  viscosity  of  sus- 
pensoids  (see  p.  151). 

When  we  deal  with  acid  albumin  it  is  found  that  the  anions  of 
the  neutral  salts  play  a  greater  role  than  do  the  cations.  Salts 
usually  lower  the  viscosity,  though  complicated  concentration 
relations  appear.  With  a  common  cation  the  anions  decrease 
the  viscosity  in  the  following  order: 

C2H302  >  S04  >  SCN  >  N03  >  Cl 

[E.  Laqueur  and  O.  Sackur  (I.e.),  W.  Frey  (l.c.),  H.  Procter,1 
L.  Zoja  (I.e.),  Wo.  Pauli  (I.e.)  and  others.] 

The  reverse  is  true  with  alkali  albumin,  where  the  cations  play 
the  chief  part.  From  a  qualitative  point  of  view  all  the  salts  bring 
about  a  decrease  in  viscosity,  but  when  the  effects  of  equal  amounts 
of  salts  are  compared  a  greater  decrease  is  noted  in  alkali  albumin 
than  in  acid  albumin.  The  salts  of  the  alkali  earth  metals  exert  a 
stronger  influence  than  those  of  the  alkali  metals. 

14.  Influence  of  Added  Substances  on  Viscosity  of  Emulsoids. 
Effects  of  Non-electrolytes  and  Mixture  of  Dispersing  Media. — 
Non-electrolytes  in  low  concentrations  usually  change  the  viscosity 
of  emulsoids  only  to  the  extent  in  which  they  increase  the  viscosity 
of  the  pure  dispersion  medium  (S.  J.  Levites,  Wo.  Pauli,  etc.). 
Yet  it  is  not  impossible  for  non-electrolytes  even  in  low  concentra- 
tions to  influence  the  viscosity  somewhat.  Thus  Handovsky 
found  that  caffeine  causes  a  very  perceptible  increase  in  the 
viscosity  of  acid  albumin.2  We  need  more  experiments  in  this 
field. 

In  greater  concentrations  the  addition  of  non-electrolytes 
causes  very  perceptible  non-additive  changes  in  viscosity.  J. 
Simon  (l.c.),  for  example,  found  alcohols,  acetone,  etc.,  to  increase 
markedly  the  viscosity  of  albumin  solutions.  In  future  studies  of 
these  phenomena  it  might  be  well  to  subtract  from  the  observed 
changes  in  viscosity  those  increases  which  result  from  mere 
mixing  of  the  alcohol  with  water.  Only  then  will  the  true  changes 
in  viscosity  due  to  the  change  in  the  colloids  themselves  be 
clearly  evidenced. 

Several  albumins,  such  as  thezeinof  Indian  corn,  are  remarkable 

1  H.  Procter,  Koll.-Zeitschr.,  3,  307  (1908). 

2  Morphine,    alcohol   in   low    concentration,    etc.,    probably   produce    similar 
effects. 


174  SPECIAL  COLLOID-CHEMISTRY 

in  that  they  dissolve  neither  in  water  nor  alcohol,  but  in  a  mixture 
of  the  two.1  It  would  be  interesting  to  study  the  viscosity  be- 
havior of  such  systems.  The  same  is  true  of  many  dyes  which 
although  soluble  in  each  of  the  pure  solvents  show  different  de- 
grees of  dispersion  and  even  different  types  of  colloidality  in 
the  two.2 

15.  Viscosity  and  Electrical  Charge  of  Disperse  Phase. — 
Nothing  is  known  as  yet  of  the  influence  of  the  electrical  charge  of 
the  disperse  phase  on  the  viscosity  of  suspensoids.  It  is  probable, 
however,  that  more  exact  measurements  will  show  the  existence 
of  such  an  influence.  We  suppose  this  because  every  electrically 
charged  particle  induces  about  it  an  electromagnetic  field  which 
hinders  its  movement  whether  such  is  "spontaneous"  or  brought 
about  from  without. 

On  the  other  hand,  E.  Laqueur  and  O.  Sackur  (I.e.),  W.  B. 
Hardy  (I.e.)  and  especially  Wo.  Pauli  (I.e.)  pointed  out  long  ago 
that  the  electric  charge  of  protein  particles  greatly  affects  the 
viscosity  of  those  solutions.  These  investigators  hold  the  elec- 
trically or  electrochemically  charged  particles  in  these  solutions 
to  spring  from  an  electrolytic  dissociation  similar  to  that  observed 
in  molecularly  dispersed,  slightly  dissociated  systems.  As  will 
become  more  evident  in  the  chapter  on  the  electrical  properties 
of  colloid  systems,  this  assumption  has  proved  both  satisfactory 
and  fruitful  in  explanation,  for  example,  of  the  variations  in 
viscosity  caused  by  added  substances.  It  may  be  said  that  when 
the  viscosity  of  a  neutral  emulsoid  rises  on  the  addition  of  some 
substance,  this  is  due  chiefly  to  an  increase  in  the  number  of  dis- 
sociated (electrically  charged)  colloid  particles.  The  correctness 
of  this  view  is  at  once  evidenced  when  we  recall  to  mind  the 
striking  increase  in  the  viscosity  of  gelatine,  soap,  or  protein 
solutions  when  small  amounts  of  acids  or  alkalies  are  added 
to  them.  The  decrease  in  viscosity  observed  in  higher  concen- 
trations of  the  acids  and  alkalies  follows  the  decrease  in  dissocia- 
tion. The  effect  of  salts  ;n  lowering  the  viscosity  of  acid-  and  alkali- 
colloids  corresponds  with  the  effect  of  salts  in  depressing  ionization 
when  a  common  ion  is  introduced.3  Table  25  (p.  171)  may  serve 

1  See  the  detailed  paper  of  G.  Galeotti  and  G.  Giampalmo,  Koll.-Zeitschr.,  3, 118 
(1908),  where  references  to  the  literature  may  also  be  found. 

2  H.  Freundlich  and  W.  Neumann,  Koll.-Zeitschr.,  3,  80  (1908). 

3  For  a  discussion  of  the  electrochemical  side  of  these  views  see  the  textbooks  of 
physical  chemistry. 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS  175 

to  show  the  general  parallelism  between  concentration  of  OH  ions 
and  viscosity.  Certain  exceptions  to  the  general  rule  are,  however, 
to  be  noted,  as  in  the  case  of  piperidine. 

The  well-grounded  fact  that  ions  are  more  strongly  hydrated 
than  electrically  neutral  undissociated  molecules  explains  why 
increase  in  dissociation  and  increase  in  viscosity  go  hand  in  hand. 
As  a  result  of  the  magnetic  field  about  the  charged  particles,  or  at 
least  through  its  increase,  we  may  imagine  the  solvent  to  be  held 
more  closely  in  the  solvent  envelopes  about  the  separate  particles. 
Thus  also  will  the  internal  friction  be  increased  and  the  separate 
particles  become  less  mobile  for  now  the  charged  particles  have 
larger  envelopes  of  the  dispersion  means  about  them.  But  let 
us  not  fail  to  point  out  that  it  does  not  seem  safe  to  say  that  this 
direct  application  of  electrochemical  laws  will,  in  the  future,  show 
itself  to  be  entirely  adequate.  But  the  ability  of  these  laws  to 
elucidate  at  least  some  of  the  complicated  relations  observed  shows 
them  to  be  at  least  partly  active. 

Future  investigators  may  reveal  great  discrepancies  between 
the  laws  governing  the  behavior  of  colloid  systems  and  the  electro- 
chemical laws  which  apply  to  molecular  and  supermolecular  dis- 
perse systems.  Notwithstanding  isolated  analogies,  colloid  sys- 
tems may  be  found  to  be  governed  by  electrochemical  laws  which 
are  not  subordinate  to  those  governing  molecular  systems  but 
coordinated  with  them.  Great  variations  from  normal  electro- 
chemical behavior  are  already  known  in  the  case  of  suspensoids.1 
We  can  discuss  these  questions  to  greater  advantage  when  we  come 
to  consider  the  electrical  properties  of  colloid  systems. 

16.  Viscosity  and  Degree  of  Dispersion;  Viscosity  of  Coarse 
and  Complex  Dispersions. — Only  a  few  observations  are  available 
on  the  theoretically  important  relation  between  degree  of  disper- 
sion and  viscosity,  and  no  systematic  study  has  as  yet  been  made 
of  any  number  of  systems  with  progressively  varying  degrees  of 
dispersion.  Theoretically  one  would  expect  the  viscosity  of  a  dis- 
persoid  to  grow  with  every  increase  in  the  amount  of  contact  sur- 
face, in  other  words,  with  the  degree  of  dispersion.  It  is  here  as- 
sumed that  the  particles  of  the  disperse  phase  move  about  with 
greater  difficulty  than  do  the  particles  of  the  dispersion  means 
itself.  The  dispersion  medium,  held  in  the  often-mentioned  sur- 
1  See  Wo.  Ostwald,  Koll.-Zeitschr.,  7,  132  (1910). 


176  SPECIAL  COLLOID-CHEMISTRY 

face  membranes,  must  have  in  addition  to  its  usual  character- 
istics a  decreased  mobility.  Experimental  evidence  can  be  cited 
to  support  this  view.  The  experiments  described  on  p.  151,  deal- 
ing with  the  decrease  in  the  viscosity  on  ageing  or  the  addition 
of  salts,  show  a  distinct  parallelism  between  decrease  in  degree  of 
dispersion  and  decrease  in  viscosity.  K.  Beck  and  K.  Ebbinghaus1 
found  that  coarse  emulsions  of  castor  oil  in  water  did  not  greatly 
change  the  viscosity  of  the  water,  but  after  gum  arabic  or  similar 
substances  had  been  added  which  permitted  the  attainment  of 
higher  dispersion,  the  viscosity  rose  considerably  above  that  of  the 
oil  or  the  pure  gum  solution.  The  increase  amounted  to  44  per 
cent.  The  fact  that  cellulose  becomes  slimy  and  viscous  with  long 
grinding  indicates  the  same  thing.  G.  Buglia2  found  milk  to  show 
a  distinct  increase  in  viscosity  after  being  "  homogenized,"  that 
is  to  say,  after  having  its  fat  finely  divided  by  being  squirted  against 
an  agate  plate.  A.  Martici3  has  studied  the  viscosity  of  oil  emul- 
sions in  soap  water  and  found  that  their  viscosity  increases  as 
the  oil  droplets  become  smaller. 

But  observations  can  also  be  cited  to  support  the  opposite 
view.  Cases  are  known  in  which  the  viscosity  increases  as  the 
degree  of  dispersion  decreases.  In  the  case  of  molecular  disper- 
soids,  it  is  the  rule  that  when  the  substances  have  a  high  mole- 
cular weight  that  they  show  a  greater  viscosity.  We  need  but 
consider  the  salts  (soaps)  of  the  homologous  fatty  acids  in  water 
(see  p.  143).  While  the  lower  members  (acetates)  change  the 
viscosity  of  water  but  little,  aqueous  solutions  of  the  higher 
members  are  solid.  The  association  of  changes  in  molecular 
weight  with  changes  in  the  viscosity  of  colloid  night-blue  solu- 
tions under  the  influence  of  changes  in  temperature  has  been 
observed  by  W.  Biltz  and  A.  von  Vegesack  (I.e.)-  They  calculated 
from  direct  osmotic  measurements  the  molecular  weight  of 
technical  night-blue  at  o°,  25°  and  50°  to  be,  respectively,  11,550, 
5260  and  3550.  A  glance  at  the  viscosity  curves  of  Fig.  27  shows 
that  the  greatest  viscosity  coincides  with  the  greatest  molecular 
weight.  The  phenomena  in  critical  fluid  mixtures  may  also  be 
used  to  show  direct  parallelism  between  viscosity  and  degree  of 

1  K.  Beck,  Zeitschr.  Physik.  Chem.,  58,  409  (1907);  K.  Ebbinghaus,  Diss.,  Leipzig, 
1907. 

2  G.  Buglia,  Koll.-Zeitschr.,  2,  353  (1908). 

3  A.  Martici,  Arch.  di.  Fisiol.,  4,  133  (1907). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  177 

dispersion.  As  first  observed  by  J.  Friedlander,1  a  mixture  of 
butyric  acid  in  water,  which  is  completely  miscible  at  higher  tem- 
peratures, shows  a  great  increase  in  internal  friction  when  cooled, 
and  this  increase  occurs  in  the  region  where  the  system  begins  to 
become  turbid,  in  other  words,  where  the  components  begin  to 
separate.  This  separation  must  of  necessity  be  highly  dispersed 
at  the  beginning  as  evidenced  by  the  fact  that  a  bluish  opalescence 
first  appears  when  the  solution  is  still  perfectly  transparent. 
The  degree  of  turbidity  and  the  viscosity  at  first  increase  steadily 
as  the  separation  proceeds.  It  is  not  impossible  that  in  this 
case  there  occurs  not  only  an  increase  in  the  number  of  drop- 
lets but  also  an  increase  in  their  size,  for  ultimately  a  coarse 
separation  of  acid  in  water  is  obtained  which  of  course  cannot 
have  occurred  suddenly.  We  can  also  cite  an  example  which 
shows  the  opposite  of  what  was  said  above  in  discussing  cellulose. 
Highly  "masticized,"  that  is  to  say,  mechanically  treated  rubber, 
yields  much  less  viscid  solutions  than  the  untreated. 

In  an  analogous  manner  the  increase  in  the  viscosity  of  emul- 
soids  with  time  during  their  gelation  indicates  a  decrease  in  their 
degree  of  dispersion. 

The  conclusion  to  be  drawn  from  these  seemingly  opposed  facts 
would  be  that,  other  conditions  being  constant,  a  dispersoid  reaches 
its  highest  viscosity  at  a  medium  degree  of  dispersion.  The  experi- 
mental verification  of  such  a  conclusion  is  a  problem  of  the  future. 

There  is  much  practical  as  well  as  theoretical  interest  attached 
to  a  comparison  of  the  viscosity  of  coarse  dispersions  with  those  of 
colloid  systems.  While  observations  on  the  viscosity  of  coarse 
suspensions2  are  few,  much  more  is  known  regarding  the  behavior 
of  coarse  emulsions.3  Of  course  in  many  of  these  experiments  we 

1  J.  Friedlander,  Zeitschr.  f.  physik.  Chem.,  38,  430(1901);  V.  Rothmund,  ibid. , 
63,  54  (1908). 

2  Besides  the  well-known  behavior  of  sand  we  may  point  out  that  M.  Franken- 
heim  [Journ.  f.  prakt.  Chem.,  54,  433  (1851)]  details  some  observations  on  increase 
in  viscosity  caused  by  the  taking  up  of  solid  particles. 

3  Besides  the  works  of  K.  Beck,  K.  Ebbinghaus,  J.  Friedlander,  V.  Rothmund, 
G.  Buglia  and  J.  Simon  we  may  also  cite  M.  Bose,  Physik.  Zeitschr.,  83,  47  (1907); 
Z.  f.  Elektroch.,  13,  499  (1907);  R.  Schenk,  Kristall.  Flussigkeiten,  32,  Leipzig,  1905; 
Eichwald,  Diss.  Marburg,  1905;  D.  Holde,  Koll.-Zeitschr.,  4,  270  (1908)  Emulsions 
of  Water  in  Mineral  Oils,  etc.;  Wo.  Ostwald,  Koll.-Zeitschr.,  6,  103  (1910);  E.  E. 
Hatschek,  ibid.,  6,  254  (1910);  7,  n  (1910);  T.  B.  Robertson,  ibid.,  7,  7  (1910); 
S.  U.  Pickering,  ibid.,  7,  n  (1910)  where  references  to  the  old  literature  may  be 
found;  M.  W.  Beyerinck  ibid.,  7,  16  (1910),  Emulsions  Consisting  of  Two  Colloids; 
F.  G.  Donnan,  Zeitschr.  f.  physik.  Chem.,  31,42  (1899);  Koll.-Zeitschr., 7.  208  (1910) 
with  H.  E.  Potts. 


12 


I78 


SPECIAL  COLLOID-CHEMISTRY 


deal  with  complex  emulsions  consisting  of  more  than  two  phases. 
Still  a  comparison  of  the  viscosity  relations  of  these  systems  with 
those  of  the  emulsoids  shows  so  many  and  at  times  such  surprising 
analogies  that  a  short  discussion  seems  valuable  especially  since  it 
serves  to  support  the  belief  that  emulsion  colloids  are  systems  hav- 
ing the  composition  liquid  +  liquid.  An  excellent  example  of  the 
increase  in  the  viscosity  of  a  liquid  when  a  second  insoluble  one  is 
emulsified  in  it,  is  offered  by  the  so-called  solid  lubricants  (engine 
grease).  Even  0.75  per  cent,  of  water  when  thoroughly  mixed  into 
liquid  solutions  of  soaps  in  mineral  oil  will  convert  these  into  salve- 
like  bodies  of  so  high  viscosity  that  they  may  be  spooned  out  in 


5       10 

Con  cen  fra  hi  on 

FIG.  32. — Influence  of  concentration  upon  the  viscosity  of  a  castor  oil- water  emulsion 
(According  to  K.  Beck.} 

coherent  masses  (D.  Holde,  I.e.).  The  same  example  serves  to 
demonstrate  the  influence  of  concentration  on  the  viscosity  of  coarse 
emulsions,  for  this  varies  within  the  concentration  limits  of  o  to 
0.75  per  cent,  water  from  that  of  a  liquid  soap  to  that  of  a  "  solid  " 
lubricant.  Another  illustration  of  the  latter  has  been  found  by  K. 
Beck  (I.e.)  and  his  coworkers  in  their  work  on  emulsions  of  acacia 
water  and  castor  oil.  While  small  amounts  of  emulsified  castor 
oil  but  slightly  increased  the  viscosity  of  the  gum  arabic  solutions 
certain  higher  concentrations  caused  sharp  increases.  Fig.  32 
illustrates  this  behavior  which  is  fully  analogous  to  that  observed 
in  emulsoids.  Excellent  analogies  for  the  great  effect  of  tempera- 
ture on  the  viscosity  of  lyophilic  colloids  can  also  be  found  in  the 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  179 

case  of  the  coarser  emulsions.  J.  Friedlander  (I.e.)  and  V.  Roth- 
mund  (I.e.)  found  the  viscosity  of  critical  fluid  mixtures  to  be 
very  sensitive  to  temperature.  The  temperature  coefficient  of 
viscosity  in  these  ranges  is  three  to  five  times  as  great  as  in  those 
in  which  the  system  has  lost  its  emulsion  nature.  The  machine 
oils  already  mentioned  may  serve  as  further  illustrative  material. 
Their  decrease  in  viscosity  with  increase  in  temperature  is  so  great 
that  one  may  distinguish  a  softening  point  and  a  dropping  point 
which  may  at  times  lie  but  one  degree  apart. 

This  indicates  that  their  viscosity  may  fall  from  that  of  a  solid 
to  that  of  a  liquid  within  the  space  of  a  few  degrees,  a  suddenness 
of  change  which  is  similar  to  that  observed  in  the  melting  points 
of  solids.  Finally,  attention  should  be  called  to  a  third  system, 
namely,  that  of  an  alcoholic  solution  of  rosin  containing  a  little 
water,  investigated  by  J.  Friedlander.  This  also  possesses  a 
relatively  large  temperature  coefficient,  namely,  one  of  5  to  6  per 
cent,  per  degree  of  temperature  against  that  of  about  2  per  cent, 
for  water. 

17.  Viscosity  and  Type  of  Disperse  Phase. — We  have  thus  far 
considered  the  viscosity  relations  of  only  the  more  common  and 
important  dispersoids,  namely,  those  having  the  composition  liquid 
+  solid  and  liquid  +  liquid.  It  should,  however,  be  remembered 
that  remarkable  increases  in  viscosity  of  a  liquid  dispersion  medium 
may  be  caused  by  finely  dividing  a  gaseous  phase  in  it  as  illus- 
trated by  the  mechanical  properties  of  foams  which  often  have 
many  of  the  characteristics  of  a  solid.  We  need  of  course  to  take 
into  account  that  strictly  two-phase  systems  of  the  type  liquid  + 
gas  are  hardly  known  and  that  the  stability  of  most  foams  is  closely 
associated  with  their  so-called  adsorption  phenomena  by  virtue 
of  which  the  gas  bubbles  condense  dissolved  substances  upon  their 
surfaces  with  consequent  formation  of  solid  films.  Yet  such 
adsorption  processes  are,  in  many  cases,  completely  reversible 
and  the  fluid  nature  of  the  membranes  is  preserved  throughout. 
Thus  saponin  foam  melts  down  to  a  homogeneous  fluid  perfectly 
free  from  coagula,  and  egg-white  may  be  freed  of  the  threads  and 
coagula  present  in  it  in  its  natural  state  by  beating  it  to  a  foam. 
The  greater  part  of  the  foam  subsequently  melts  down  to  a  solu- 
tion perfectly  free  from  flocculi.  This  is  evidence  for  the  fluid 
nature  of  the  walls  of  the  foam.  The  preparation  and  detailed 


l8o  SPECIAL  COLLOID-CHEMISTRY 

investigation  of  colloid  foams  would  evidently  be  of  great  interest 
to  general  colloid  chemistry.1 

If  one  compares  the  internal  friction  of  the  three  typical  dis- 
persoids  having  a  fluid  dispersion  medium,  it  is  found  that  a  low 
initial  viscosity  of  disperse  phase  by  no  means  precludes  the  at- 
tainment of  high  viscosity  values  for  the  whole  system.  In  fact,  if 
colloid  dispersoids  are  compared  with  each  other,  it  is  found  that 
emulsoids  usually  exhibit  a  higher  viscosity  than  the  suspensoids 
having  the  same  concentration  and,  in  view  of  the  great  stability 
of  highly  dispersed  foams,  it  even  seems  as  though  such  when  in 
a  colloid  degree  of  dispersion  might  show  still  higher  viscosity 
values.  We  must  of  course  distinguish  between  high  viscosity  and 
the  value  of  other  physical  properties  such  as  hardness.  Paradox- 
ical as  it  may  seem,  it  even  appears  as  though  viscosity  of  the  dis- 
persion medium  and  viscosity  of  the  disperse  phase  may  be  only 
of  indirect  significance,  for  it  seems  probable  that  the  properties 
of  the  different  surfaces  (liquid-solid,  liquid-liquid,  and  liquid- 
gaseous)  and  not  the  low  viscosity  value  of  the  disperse  phase 
itself  are  primarily  responsible  for  the  viscosity  of  the  dispersoid 
as  a  whole. 

§26.  Surface  Tension  of  Colloid  Solutions 

i.  General  Remarks.- — A  closed  two-phase  dispersoid  has  a 
series  of  surfaces.  The  most  important  is  the  one  between  the 
disperse  phase  and  the  dispersion  medium.  There  is,  in  addition, 
the  surface  between  the  whole  dispersoid  and  its  surroundings,  in 
considering  which  we  must  distinguish  between  the  surface  bound- 
ing the  dispersoid  and  its  vapor  and  that  between  the  dispersoid  and 
the  walls  of  the  vessel.  If  we  remember  that  there  are  two  surface 
energies  in  every  surface,  then  we  may  distinguish  six  different 
surface  tensions.  If  we  consider  that  the  disperse  particles  may 
also  come  in  contact  with  both  the  gaseous  boundary  and  the 
walls  of  the  vessel  (as  is  actually  the  case  in  the  adsorption 
phenomena  occurring  in  three-phase  systems),  the  number  of 
tensions  to  be  considered  is  increased  to  ten,  while  in  three-phase 
dispersoids  the  number  rises  to  eighteen.  We  cannot  say  in  ad- 

1  For  some  observations  on  fine  foams  see  Wo.  Ostwald,  Koll.-Zeitschr.,  I,  333 
(1907).  Systems  belonging  to  this  class  are  also  described  by  Schroeder,  Poggen- 
dorf's  Ann.,  137,  76  (1869);  see  also  the  patent  of  J.Weinmayr,  described  in  Chem. 
Centralbl.,  586  (1910). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS      l8l 

vance  that  this  or  that  surface  tension  is  insignificant  in  determin- 
ing the  characteristics  of  a  dispersoid  or  a  colloid.  The  expansile 
surface  tension  between  the  dispersion  medium  and  vessel  walls, 
for  example,  determines  its  ability  to  "wet"  the  surface;  while  the 
relation  of  positive  to  negative  surface  tension  between  the  dis- 
perse phase  and  dispersion  medium  determines  the  degree  of  dis- 
persion (see  p.  81).  Other  groups  of  tension  are  responsible 
for  the  processes  of  coagulation,  adsorption,  etc.  At  the  present 
time,  however,  the  sense  and  value  of  only  a  few  of  these  tensions 
are  known;  in  fact  quantitative  measurements  are  available  of 
but  a  single  surface  tension,  namely,  that  of  the  positive  tension  in 
the  surface  between  the  dispersoid  and  its  vapor. 

2.  Experimental  Facts. — Investigations  show  that  the  positive 
surface  tension1  of  a  colloid  solution  at  its  free  surfaces  may  be 
more,  or  less,  or  equal  to  that  of  the  pure  dispersion  medium 
(Rayleigh,2  A.  Pockels,3  W.  Ramsden,4  G.  Quincke,5  H.  Picton  and 
S.  E.  Linder,6  L.  Zlobicki,7  W.  Frei,8  G.  Buglia,9  F.  Bottazzi  and 
C.  Victorow10).  Usually  the  tension  is  less. 

The  surface  tension  of  water  is  increased  by  gum  arabic,  starch 
and  plum  gum.  It  is  lowered  by  gelatine,  glue,  egg-albumin,  dex- 
trin, cherry  and  sweet  cherry  gum.  It  is  greatly  lowered  by  fats, 
fatty  acids,  soaps,  resins,  tannic  acid,  etc.  Tables  26  and  27  taken 
from  G.  Quincke  and  L.  Zlobicki  may  serve  in  illustration. 

Both  the  increase  or  the  decrease  in  surface  tension  follows  the 
concentration  of  the  colloid.  Traces  of  fatty  acids,  of  soaps,  etc., 
suffice  to  lower  greatly  the  surface  tension  of  water  as  seen  in  Table 
26.  The  surface  tension  of  colloid  solutions  as  of  liquids  in  general 
decreases  as  the  temperature  rises  but,  as  Table  27  shows,  is  much 
more  marked  than  in  the  case  of  the  pure  dispersion  medium  alone. 

1The  textbooks  of  physics  and  physical  chemistry  should  be  consulted  for 
methods  of  measuring  the  positive  surface  tension. 

2  Rayleigh,  Proc.  Roy.  Soc.,  47,  364  (1890). 

3  A.  Pockels,  Nature,  46,  418  (1892);  Drude's  Ann.  d.  Physik.,  8,  (1902). 

4W.  Ramsden,  Engelmann's  Arch.  f.  Anat.  und  Physiol.  Abt.  f.  Physiol.,  517 
(1894);  Z.  f.  physik.  Chem.,  47,  341  (1902);  Proc.  Roy.  Soc.,  72,  156  (1904). 

6  G.  Quincke,  Wiedemann's  Ann.,  35, 582  (1888)  Ber.  d.  Berl.  Akad.  d.  Wissensch., 
38,  493,  858  (1901);  Drude's  Ann.  d.  Physik.,  7,631  (1901); ibid.,g, 969  (1902);^., 
10,  507  (1903);  ibid.,  ii  (1904). 

6  H.  Picton  and  S.  E.  Linder,  Journ.  Chem.  Soc.,  87,  1924  (1905). 

7L.  Zlobicki,  Bull.  Acad.  Sc.  Cracovie,  Juli,  488  (1906). 

8  W.  Frei,  Zur  Theorie  der  Hamolyse,  Diss.,  Zurich,  1907;  Transvaal  Medic. 
Journ.,  August,  1908. 

9  G.  Buglia,  Biochem.  Zeitschr.,  n,  311  (1908). 

10  F.  Bottazzi  and  C.  Victorow,  Rend.  R.  Ac.  Line.,  19,  659  (1910). 


1 82  SPECIAL  COLLOID-CHEMISTRY 

TABLE  26. — SURFACE  TENSIONS  OF  COLLOID  SOLUTIONS  AT  ABOUT  20° 
(According  to  G.  Quincke) 


Substance 

Specific  gravity 

Surface  tension  against  "  air" 

Water  

I    OOOO 

8o<  ? 

Egg-albumin 

I    036? 

Aqueous  bile  solution  (9%) 

1.0384  1 
I  .0384  1 

I     IOI33 

•yo4 
j  5-370  to 

I  4.913 
5076 

Venetian  Soap 
^inno  per  cent..  . 

o  0083 

2  681 

/^oo  Per  cent. 

O   OOO2 

2    672 

1  Y±o   per  cent  ."* 
Tannic  acid,  10  per  cent  

I  .  OOOQ 
I  .0^12 

2.563 
"?   8<7 

Gum  arabic,  20  per  cent..  
Isinglass      } 
Gelatine       f  very  dilute  

1.0708 
I  .  OOOO 
I    OOOO 

7.603 
6.790 

7    272 

Ag»r             J 

I  .  OOOO 

7.842 

TABLE  27. — SURFACE  TENSIONS  OF  COLLOID  SOLUTIONS 
(According  to  L.  Zlobicki) 


a  Grams  gelatine  in  100  cc.  solution 

2  Grams  gum  arabic  in  100  cc.  solution 

Temp. 

Surface  tension  in  mg.  /mm. 

Temp. 

Surface  tension  in  mg.  /mm. 

Solution 

Water 

Solution 

Water 

o.o 

"•3 
17.0 

6.62 
6.21 

5.98 

7.69 
7.52 
7-43 

0.0 

6.6 
17.0 

8.66 
8.47 
8.16 

7.69 

7-59 

7.42 

24-5 

5-70 

7.32 

24.0                 7.75 

7-33 

The  type  of  the  disperse  phase  is  of  particular  importance  in 
determining  the  change  in  the  surface  tension  of  the  pure  dispersion 
medium.  This  is  indicated  by  the  fact  that  all  the  above-men- 
tioned examples  are  emulsoids.  Coarse  suspensions  and  suspensoids 
hardly  alter  the  surface  tension  of  the  dispersion  medium.  H. 
Picton  and  S.  E.  Linder  (I.e.)  found  suspensoid  arsenious  tri- 
sulphide,  even  in  concentrations  of  2  per  cent.,  and  dilute  iron  hy- 
droxide to  produce  so  minimal  a  decrease  in  the  surface  tension 
of  pure  water  that  it  scarcely  exceeds  the  experimental  error. 
N.  Sahlbom1  obtained  analogous  results.  L.  Zlobicki  (I.e.)  found 
coarse  aqueous  suspensions  of  emery,  mastic  and  gamboge  and 

1N.  Sahlbom,  Kolloidchem.  Beih.,  2,  No.  3  (1910). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS 


183 


colloid  suspensions  of  silver  and  platinum  to  have  the  same  surface 
tension  as  the  pure  dispersion  medium.  The  temperature  coeffi- 
cient of  the  surface  tension  of  these  systems  was  also  the  same  as 
that  of  the  pure  dispersion  means. 

These  results  seem  to  show  that  only  emulsoids  decrease  the 
surface  tension  of  their  dispersion  media.  This  difference  can,  in 
fact,  be  used  as  a  means  for  distinguishing  the  two  classes  of  col- 
loids from  each  other.  Here  again  is  evidenced  the  close  connec- 
tion between  emulsoids  and  molecular-dispersoids  in  that  the 
latter  also  always  exhibit  a  surface  tension  different  from  that  of 
their  pure  solvents. 

As  already  indicated  (p.  55),  the  same  chemical  substance 
may  assume  either  emulsoid  or  suspensoid  properties  in  dif- 
ferent dispersion  media.  Thus  soaps,  many  dyes,  etc.,  form  emul- 
soids in  water  while  they  form  suspensoids  in  alcohol.  One 
would  expect  this  distinction  to  show  itself  in  the  surface  tension 
behavior  of  the  different  solutions  when  compared  with  that  of 
their  pure  dispersion  media,  which,  in  fact,  it  does,  as  H.  Freund- 
lich  and  W.  Neumann1  have  found.  Table  28  illustrates  this  in- 
teresting fact  in  that  it  shows  that  the  surface  tensions  of  the 
aqueous  dispersion  media  are  noticeably  decreased  while  those  of 
the  alcoholic  solutions  show  no  change,  or  if  anything,  a  slight  in- 
crease. 

TABLE  28. — SURFACE  TENSIONS  OF  COLLOID  DYES  IN  WATER  AND  IN  ALCOHOL 
(According  to  H.  Freundlich  and  W.  Neumann)  * 


Water 
Substance 

Surface  tension 

Alcohol 
Substance 

Surface  tension 

Water  

7<C.o 

Alcohol  

21  .0 

Night-blue  

68.3 

Night-blue  

22    3 

« 

Congo  red  
Crystal  violet  
New  fuchsin  . 

67.4 
74.8 
72.7 
7«r    y 

H 

Crystal  violet  
u 

New  fuchsin  . 

22.2 
22  .2 
22.2 
22    2 

Diamond  fuchsin  
Rhodamin**  

74.6 

74.  A 

u 

Rhodamin  .  . 

22.1 
21    O 

n 

22.9 

1  H.  Freundlich  and  W.  Neumann,  Koll.-Zeitschr.,  3,  80  (1908). 
*  The  surface  tensions  were  measured  by  the  rise  in  capillary  tubes. 
**  This  dye  is  probably  molecularly  dispersed. 


184  SPECIAL  COLLOID-CHEMISTRY 

The  surface  tension  of  emulsoids  is  changed  by  the  addition  of 
dispersed  substances.  This  was  to  be  expected  from  their  effect 
on  viscosity.  The  addition  of  small  quantities  of  hydroxyl  ions 
raises,  while  the  addition  of  hydrogen  ions  decreases  the  surface 
tension  of  neutral  gelatine  or  neutral  blood  serum  (G.  Buglia, 
W.  Frei).  F.  Bottazzi  and  C.  Victorow  observed  NaOH  to  affect 
greatly  the  surface  tension  of  soap  solutions,  a  behavior  which  was 
the  image  of  the  corresponding  one  regarding  viscosity.  Very 
low  concentrations  caused  a  great  decrease  in  tension;  higher 
concentrations  led  to  an  increase  which  soon  reached  a  maximum 
to  give  way  to  a  second  more  gradual  decrease.  W.  Frei  found 
the  anions  SO4,  Cl,  N03  and  the  cations  Na,  K,  Mg  and  Ca  to 
increase  the  surface  tension  of  neutral  gelatine  solutions  in  almost 
the  same  order  in  which  they  increase  that  of  pure  water.  It  is 
an  interesting  fact  that  the  order  of  the  anions  is  reversed  depend- 
ing upon  whether  the  gelatine  is  acid  or  alkaline. 

Several  interesting  investigations  show  how  varied  and  compli- 
cated are  the  relations  when  we  also  consider  the  changes  in  the 
other  surface  tensions  of  the  dispersoid,  especially  if  it  is  of  the 
type  fluid  +  fluid  in  which  we  are  able  to  measure  experimentally 
the  surface  tension  between  the  disperse  phase  and  dispersion 
medium.  According  to  G.  N.  Antonow1  and  W.  C.  McC.  Lewis2 
we  know  of  two  cases,  namely,  water-ether  and  aqueous  glyco- 
colate  solution-mineral  oil,  in  which  the  surface  tension  at  the 
interfaces  increases  with  rising  temperature  instead  of  decreasing 
as  is  usually  the  case.  Even  though  this  was  observed  in  the  case 
of  but  slightly  dispersed  systems,  there  is  no  reason  for  not  be- 
lieving that  temperature  exerts  a  like  influence  in  highly  dispersed 
systems  of  the  same  composition.  The  decrease  in  capillary 
rise,  with  increasing  temperature,  observed  by  R.  Schenck,3  in 
cholesteryl-benzoate  (a  so-called  crystalline  liquid)  may  come 
under  this  head. 

We  need  now  to  call  attention  to  a  factor  which  must  be 
considered  in  measuring  the  surface  tension  of  molecularly  dis- 
persed systems  but  which  assumes  a  still  greater  importance  in 
colloid  systems.  Willard  Gibbs  has  formulated  a  theorem  which 
has  been  confirmed  at  least  qualitatively  by  other  investigators. 

1  G.  N.  Antonow,  Journ.  Chim.  physique,  5,  372  (1907). 

*  W.  C.  McC.  Lewis,  Philos.  Mag.,  15,  506  (1908). 

3  R.  Schenck,  Kristallinische  Fliissigkeiten,  1 1 29,  Leipzig,  1901. 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  185 

It  states  that  substances  which  lower  the  surface  tension  of  the 
pure  dispersion  medium,  tend  to  collect  in  its  surface.  Because  of 
this  rise  in  concentration  the  surface  tension  must,  with  time,  be 
come  progressively  lower  wherefore  it  is  conceivable  that  it  may, 
under  certain  circumstances,  attain  a  value  different  from  the 
original  present  in  the  surface  immediately  after  its  formation. 
The  latter  surface  tension,  which  can  be  measured  only  on  freshly 
formed  or  constantly  renewed  surfaces  is  called  the  dynamic 
surface  tension;  that  which  is  present  after  some  time,  the 
static.  The  distinction  between  these  two  surface  tensions  is  of 
especial  importance  in  colloid  solutions  because  such  very  small 
amounts  of  many  colloid  substances  are  able  to  decrease  so  greatly 
the  surface  tension  of  the  pure  dispersion  medium.1 

1  See  the  recent  work  of  Wm.  C.  McC.  Lewis,  Z.  f.  physik.  Chem.,  74,  619  (1910) 
in  which  are  detailed  the  surface  tensions  of  colloid  solutions  against  their  own  vapors 
and  against  the  surface  of  various  liquids. 


CHAPTER  VI 

MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 

m.  MOVEMENT  IN  COLLOID  SYSTEMS  AND  ITS  RESULTS 

§27.  Brownian  Movement 

i.  General  Remarks. — The  Fundamental  Phenomenon. — 
Literature. — All  dispersoids  of  a  sufficiently  great  degree  of  dis- 
persion and  hav'ng  a  fluid  or  gaseous  dispersion  medium,  show 
under  the  microscope  and  ultramicroscope  a  characteristic  move- 
ment. This  was  discovered  by  the  English  botanist,  R.  Brown,1 
and  has  been  named  for  him.  Brownian  movement  is  also  known 
as  "spontaneous"  or  "molecular"  movement  though  the  latter 
term  should  be  used  cautiously.  The  separate  particles  of  the 
disperse  phase  exhibit  a  trembling  and  rotary  movement  and  when 
the  particles  are  very  small,  as  in  colloid  solutions,  the  movement 
has  been  described  by  Zsigmondy  as  "dancing,  hopping  and 
skipping"  in  nature  and  also  as  " translatory "  and  "progressive." 
The  movement  of  the  smaller  particles  differs  from  that  of  the 
larger  (microscopic)  ones  in  that  the  former  travel  along  straight 
lines  and  suddenly  change  their  direction  while  the  latter  follow  a 
more  curved  path. 

The  movements  do  not  occur  in  one  plane  only  but  in  all  direc- 
tions. As  one  observes  the  "optical  cross  section"  of  a  prepara- 
tion either  microscopically  or  ultramicroscopically,  one  sees  the 
individual  particles  disappear  and  reappear  as  they  move  down- 
wards and  upwards. 

Many  pictures  of  this  characteristic  movement  have  appeared. 
In  Figs.  33,  34.  35,  36  and  37  are  reproduced  some  particularly 
characteristic  types  of  the  movement  according  to  V.  Henri,2  R. 

1  R.  Brown,  Philos.  Mag.  (i),4,  101  (1828);  6,  161  (1829);  8,41  (1830);  and  also 
Poggendorf's  Ann.  d.  Physik.,  14,  29  (1828). 

2  V.  Henri,  Compt.  rend.,  147,  62  (1908).    A  review  of  the  subject  of  Brownian 
movement,  particularly  as  illustrated  in  the  movements  of  the  spherules  of  liquid 
rubber,  may  be  found  in  his  Le  Caoutchouc  et  la  Guta-percha,  2405,  1906  and  1908. 

1 86 


MECHANICAL   PROPERTIES   OF   COLLOID    SYSTEMS  187 

Zsigmondy1  and  0.  Lehmann.2  In  picturing  such  movement  in 
but  one  plane  we  can,  of  course,  show  only  the  projections  of  the 
paths  of  the  particles.  To  a  discussion  of  more  exact  methods  of 
determining  and  measuring  these  movements  we  shall  return  later 
(p.  192). 

Examples  of  dispersions  exhibiting  Brownian  movement  are 
suspensions  of  gutta-percha,  mastic,  etc.,  prepared  by  adding  water 
to  very  dilute  alcoholic  solutions;3  suspensions  of  ultramarine, 
cinnabar,  carmine,  etc.,  in  which  the  disperse  phase  is  amorphous 
or  cryp to-crystalline;  the  contents  of  the  chalk  sacs  to  be  found  on 
either  side  of  the  spine  in  the  frog  and  in  which  the  disperse  phase 


FIG.  33. — Brownian  movement  in  milk.     (According  to  0.  Lehmann.} 

consists  of  definite  prismatic  crystals;  metal  hydrosols;  metal 
sulphide  hydrosols  and  other  suspensoids.  Animal  and  vegetable 
milks  [0.  Lehmann,  V.  Henri,  (I.e.}]  are  examples  of  systems  having 
a  liquid  disperse  phase  and  showing  B  ro wnian  movement.  B  ro wn- 
ian  movement  may  also  be  observed  in  gas-solid  dispersoids  such 
as  tobacco  smoke,  cooling  ammonium  chloride  vapors  and  con- 
densing metal  vapors.  It  may  also  be  observed  in  gas-fluid 
dispersoids  as  in  fog. 

Strong  magnification  is  usually  necessary  to  observe  Brownian 
movement.  Dark  field  illumination  together  with  ultramicro- 
scopic  methods  are  especially  suited  for  the  examination  of  colloids. 

1  R.  Zsigmondy,  Z.  Erkenntnis  d.  Kolloide,  106,  Jena,  1905. 

2  O.  Lehmann,  Mplekularphysik.,  i,  264,  Leipzig,  1888. 

3  See  J.  Perrin,  Die  Brownsche  Bewegung  und  die  wahre  Existenz  der  Molekiile 
(Dresden  1910)  for  methods  of  preparing  suitable  suspensions  for  the  observation  of 
the  movement. 


i88 


SPECIAL  COLLOID-CHEMISTRY 


FIG.  34. — In  a  neutral  medium. 


.» 


FIG.  35. — In  an  alkaline  medium.  FIG.  36. — In  an  acid  medium. 


FIG.  37. 

FIGS.  34  TO  37. — Brownian  movement.  A,  B  and  C  are  drawn  from  cinemato- 
graphic photographs  of  gutta-percha  particles.  D  represents  the  translatory 
Brownian  movement  of  a  gold  particle  having  a  diameter  of  about  10  /z^t.  (Accord- 
ing to  R.  Zsigmondy.}  Figures  A,  B  and  C  are  enlargements  of  i  to  34,000;  figure  D 
one  of  i  to  5000. 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  189 

H.  Molisch1  has  shown  how,  under  favorable  conditions,  this 
movement  may  be  detected  with  the  naked  eye.  The  juice  of 
the  milkweed  (Euphorbia)  is  especially  adapted  for  this.  A  drop 
of  the  material  is  placed  upon  a  slide  and  held  at  a  good  visual 
distance  in  a  vertical  or  almost  vertical  position  while  sunlight 
or  the  concentrated  light  from  an  arc  is  allowed  to  fall  upon 
it  at  a  slight  angle.  When  properly  placed  "the  molecular  move- 
ment (Brownian  movement)  of  the  resin  particles  appears  in  the 
form  of  a  peculiar  trembling,  a  lively  dance,  and  a  swarming 
of  the  microscopic  particles  giving  rise  to  a  beautiful  play  of 
colors.  Finely  ground  india  ink  in  water  may  also  be  recom- 
mended for  this  experiment"  (H.  Molisch,  I.e.}. 

Directions  for  observing  Brownian  movement  with  the  aid  of 
a  projection- apparatus  have  been  suggested  by  J.  Perrin  (I.e.). 

Since  its  discovery  in  1827,  Brownian  movement  has  been 
much  investigated  both  experimentally  and  theoretically.  The 
rather  comprehensive  literature,  for  it  comprises  more  than  100 
articles,  cannot  be  cited  here.  We  shall  refer  to  some  specific 
articles  only;  for  reviews  of  the  subject  the  reader  must  look 
elsewhere.2 

2.  The  Independence  of  Brownian  Movement  of  External 
Sources  of  Energy. — When  one  tries  to  account  for  the  forces 
responsible  for  these  remarkable  movements  one  is  at  first  inclined 
to  think  them  due  to  the  effect  of  external  agencies  such  as  external 
vibrations,  differences  in  temperature,  etc.,  due  to  unequal  illu- 
mination, evaporation,  surface  tension  movements,  chemical 
changes,  etc.  Chr.  Wiener3  and  G.  Gouy4  are  to  be  especially 
mentioned  of  those  who  have  made  critical  investigations  to  show 
that  none  of  these  factors  are  responsible  for  Brownian  movement. 
We  cannot  go  into  a  detailed  restatement  of  the  many  experiments 

1  H.  Molisch,  Koll.-Zeitschr.  2,  Suppl.  I,  9  (1907);  Zeitschr.  f.  wissensch.  Mikros., 
23,  97  (1907);  Sitz.  Ak.  Wiss.  Wien,  116,  Abt.  i,  Marz.,  1907. 

2  See,  for  example,  O.Lehmann,  Molekularphysik.,  i,  264,  Leipzig,  1888,  where  a 
detailed  review  of  the  older  papers  up  to  1888  may  be  found;  The  Svedberg,  Nov. 
Act.  Soc.  Sc.  Upsaliensis,  Ser.  IV,  2,  125  (1907)  where  50  articles  are  referred  to; 
Koll.-Zeitschr.,  7,  i  (1910),  where  references  to  newer  work  and  methods  of  observa- 
tion are  found;  J.  Perrin,  Kolloidch.  Beih.,  I,  Heft  6-7  (1910)  also  available  in  mono- 
graph form  and  dealing  particularly  with  French  workers;  W.  Mecklenburg  Die 
experimentelle  Grundlegung  der  Atomistik,  Jena,  1910,  etc. 

3  Chr.  Wiener,  Poggendorf's  Ann.,  118,  79  (1863). 

4  G.  Gouy,  Journ.  de  Physique,  2  Ser.,  7,  561  (1888);  Compt.  rend.,  109,  102 
(1889);  Revue  generate  des  Sciences,  i,  1895. 


SPECIAL  COLLOID-CHEMISTRY 

that  prove  the  fundamental  independence  of  Brownian  movement  of 
external  sources  of  energy.  Only  the  following  points  are  mentioned 
as  of  particular  importance.  In  connection  with  them  it  must  be 
kept  in  mind  that  all  the  various  factors  mentioned  of  course, 
influence  the  extent  of  Brownian  movement  but  neither  positive  nor 
negative  variations  in  them  are  capable  of  suppressing  entirely  the 
movement  inherent  in  the  particles  themselves. 

The  r61e  of  vibration,  changes  in  temperature,  evaporation, 
etc.,  in  modifying  Brownian  movement,  may  be  shut  out  by 
working  in  basements,  mines  and  fields  (G.  Gouy,  Chr.  Wiener), 
with  water  baths  and  with  sealed  containers,  etc. 

The  independence  of  Brownian  movement  of  light  effects  may 
be  proved  by  working  with  different  kinds  of  light  from  which  the 
heat  waves  have  been  carefully  excluded  [G.  Gouy,  R.  Zsigmondy 
(l.c.,  1905)]- 

That  mutual  attractions  and  repulsions  of  "swinging"  par- 
ticles, dependent,  for  example,  upon  differences  in  electrostatic 
charge,  are  not  the  cause  of  Brownian  movement  has  been  shown 
by  Chr.  Wiener  (l.c.)  and  C.  Fuchs.1 

The  Svedberg  (I.e.)  has  shown  on  silver  hydrosols  that  neither 
neutralization  of  the  charge  nor  its  reversal,  as  may  be  brought 
about  through  the  addition  of  traces  of  electrolytes,  have  any  effect 
upon  the  velocity  of  the  particles.  The  amount  and  sense  of  the 
electric  charge  of  such  particles  may  be  determined  from  their 
migration  in  an  electric  field.  The  following  table  illustrates  the 
independence  of  Brownian  movement  of  the  sense  and  amount  of 
the  electric  charge  as  determined  by  measuring  the  value  of  2 A ,  of 
the  significance  of  which  more  will  be  said  later  (see  page  193). 


TABLE  29. — INDEPENDENCE  OF  BROWNIAN  MOVEMENT  OF  THE  SENSE  AND  SIZE  OF 
THE  ELECTRIC  CHARGE  OF  COLLOID  SILVER  PARTICLES 

(According  to  The  Svedberg) 

Sense  and  size  of  electric  charge  determined  and 

measured  from  the  speed  of  migration  of  the 

particles,  M  /seconds:  volt  /cm. 

+2.10  2.5 

+  0.26  2.5 

—  0.42  2.4 

-1.76  2.4 

1  C.  Fuchs,  Rep.  d.  Physik.,  25,  735  (1889). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  IQI 

J.  Perrin  (I.e.,  1910,  273)  has  also  found  that  the  addition  of 
traces  of  acids  to  gutta-percha  suspensions,  which  first  neutralize 
and  then  reverse  the  sense  of  their  original  charge,  has  "no  appre- 
ciable" influence  on  their  Brownian  movement. 

The  movement  is  not  caused  or  markedly  influenced  by  any 
chemical  reactions  occurring  between  disperse  phase  and  dispersion 
means.  This  is  proved  not  only  by  the  fact  that  all  chemically 
heterogeneous  substances  thus  far  investigated  exhibit  the  move- 
ment when  sufficiently  dispersed,  but  by  the  further  fact  that  the 
intensity  of  the  Brownian  movement  in  a  given  dispersoid  is 
always  the  same,  in  other  words,  does  not  change  with  ageing.  If 
chemical  reactions  were  responsible  for  the  movements,  say  indi- 
rectly through  changes  in  capillarity  (as  in  the  case  of  mercury 
droplets  in  contact  with  sulphuric  acid,  and  potassium  bichromate) 
then  the  movements  would  cease  after  a  time.  As  a  matter  of  fact, 
solid  particles  and  gas  bubbles,  imbedded  in  many  minerals,  and 
therefore  of  course,  very  old,  show  Brownian  movement.  (See  for 
example  G.  Gouy,  I.e.}.  Like  considerations  exclude  all  the 
capillary  theories1  of  Brownian  movement  which  at  first  glance 
are  so  plausible.  At  present,  it  is  inconceivable  why  in  a  closed 
system,  an  equilibrium  should  not  ultimately  become  established 
between  the  participating  surface  tensions  for  example. 

Finally,  it  should  be  mentioned  that  the  type  of  the  disperse 
phase  or  of  the  dispersion  means  does  not  make  Brownian  move- 
ment possible  provided  the  system  is  from  the  outset  of  a  kind  to 
permit  it,  in  other  words,  is  either  liquid  or  gaseous.  Not  only 
solid  and  liquid  particles  but  gaseous  ones  as  well  show  Brownian 
movement  in  fluids.  But  solid  and  liquid  particles  show  Brownian 
movement  in  gases  also,  as  in  smoke,  in  condensing  metallic 
vapors,  in  fog,  etc.2  (L.  J.  Bodaszewski,3  H.  Molisch,  (I.e.),  F. 
Ehrenhaft,4  M.  de  Broglie.5)  That  Brownian  movement  is  inde- 
pendent of  the  type  of  the  disperse  phase  also  proves  that  density 

1  Such  capillary  theories  of  Brownian  movement  have  been  set  up  by  G.  van  der 
Mensbrugghe,  Poggendorf's  Ann.  138,  323  (1869);  C.  Maltezos,  Ann.  chim.  phys. 
(?)  Ji  $$91*804);  Compt.  rend.  121,303(1895);  G.  Quincke,  Verh.  d.  Naturforscher 
usw.,  26,  Diisseldorf,  1898;  Beibl.  Ann.  d.  Physik.  23,  934  (1899)  etc.,  and  others. 

2  O.  Lehmann,  Molecular  physik,  2,  5,  Leipzig,  1888. 

3L.  J.  Bodaszewski,  Dingler's  Polytechn.  Journ.,  239,  325  (1881). 

4  F.  Ehrenhaft,  Sitz.  Ak.  Wiss.  Wien,  116,  1139,  (1907),  ibid  Marz.  1909;  Physik. 
Zeitschr.  12,  308  (1909),  etc. 

6  M.  de  Broglie,  148,  1165,  1315  (1906);  Le  Radium  203,  (1909). 


SPECIAL  COLLOID-CHEMISTRY 

is  not  of  fundamental  importance  in  its  causation  as  already 
emphasized  by  the  earlier  writers,  Chr.  Wiener,  G.  Gouy,  etc. 

These  investigations,  often  of  a  most  painstaking  nature,  show 
that  the  source  of  energy  for  Brownian  movement  lies  within  the 
disperse  system  itself  and  is  obviously  of  a  very  general  nature 
for  it  evidences  its  effects  under  the  most  varied  external  condi- 
tions. Brownian  movement  is,  however,  observed  only  in  dis- 
perse systems,  more  particularly  only  in  such  as  have  a  high  degree 
of  dispersion.  The  kinetic  hypothesis  according  to  which  gases 
and  liquids  are  regarded  as  conglomerates  of  rapidly  moving 
molecularly  dispersed  particles,  has  recently  been  applied  to 
Brownian  movement.  In  fact,  some  have  seen  in  this  direct 
evidence  for  the  correctness  of  .the  kinetic  theory  as  applied,  say 
to  the  movement  of  liquid  particles.  We  return  to  this  question 
on  page  205.  While  really  marvelling  at  the  successful  applica- 
tions that  have  been  made  of  this  kinetic  hypothesis,  it  seems 
to  me  not  impossible  that  future  investigations  may  yield  another 
more  universal  and  less  hypothetical  explanation  of  this  spon- 
taneous movement. 

3.  More  Exact  Determination  and  Measurement  of  Brownian 
Movement. — Various  methods  have  been  devised  for  the  exact 
quantitative  study  of  this  very  irregular  movement.1  Evidently 
graphic  representations  in  one  plane  can  only  show  a  part  of  the 
movement.  It  may  be  assumed,  however,  that  the  movement  in 
all  directions  is  of  the  same  nature.  The  paths  of  the  Brownian 
movement  of  isolated  particles  have  been  traced  by  F.  M.  Exner 
(I.e.).  He  equipped  his  microscope  with  a  drawing  apparatus 
and  followed  the  movements  of  the  particles  on  a  smoked  glass 
with  a  needle.  If  the  time  required  for  a  particle  to  traverse  a 
certain  path  is  noted  with  a  stop  watch  and  the  path  is  then  meas- 
ured one  obtains,  by  division,  the  average  velocity  of  the  particle. 

Another  ingenious  method,  devised  by  The  Svedberg,2  is 
based  on  the  following  principle.  When  one  allows  a  fluid  dis- 
persoid  to  flow  at  constant  velocity  and  with  sufficient  speed 
through  the  field  of  a  microscope  or  ultramicroscope  one  observes 
a  whole  series  of  light  curves.3  These  are  the  optical  after-images 

1  See  especially  the  critical  presentation  of  The  Svedberg,  Koll-Zeitschr.  7,  i  (1910) ; 
J.  Perrin,  I.e.',  St.  Jahn,  Jahrb.  f.  Radioakt.  16,  235  (1909). 

2  The  Svedberg,  I.e.,  also  Z.  f.  Elektroch,  12,  853,  909  (1906). 
8  Dark  ground  illumination,  must,  of  course,  be  used. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


193 


of  the  individual  dispersed  particles  which  themselves  move  too 
rapidly  to  be  seen.  The  curves  have  a  wave-like  or  zigzag  form 
as  shown  in  Fig.  38.  Their  deviation  from  the  horizontal  evidently 
is  a  measure  of  the  intensity  of  oscillation  occasioned  by  the  spon- 
taneous motion  of  the  particles.  The  height  of  the  crests  or  the 
amplitude  (=  A)  may  be  measured  directly  with  a  micrometer,  or 
be  estimated.  When  the  rate  of  flow  is  known,  the  average  ab- 
solute velocity  of  a  particle  may  be  calculated1  after  the  constant 


FIG.  38. — Diagram  illustrating  the  measurement  of  Brownian  movement. 

of  the  apparatus  itself  has  been  determined.2  With  the  fluid  at 
rest,  iA  corresponds  to  the  oscillation  of  the  particle  about  its 
initial  position. 

Still  more  exact  measurements  may  be  obtained  by  photo- 
graphic and  especially  by  kinematographic  means.  With  these, 
which  M.  Seddig3  V.  Henri  (I.e.),  H.  Siedentopf4  and  The  Svedberg 
(I.e.,  1910)  have  used  in  different  ways,  the  change  in  position  of 
the  particles  with  time,  in  other  words,  their  oscillations,  may  be 
accurately  measured.  Seddig,  Henri,  Chaudesaigues,  etc.,  deter- 
mined the  change  of  position  of  a  particle  by  photographing  a 
preparation  at  short  intervals.  Seddig  photographed  it  twice 
every  Jj/fo  second,  Henri,  with  kinematographic  apparatus,  on  a 
moving  film  several  times  every  ^0  second.  The  interesting  ap- 
paratus of  the  last-named  investigator  is  shown  in  Fig.  39.  To 
the  left  is  the  kinematographic  camera,  in  the  middle,  the  ultra- 

1  For  details  regarding  these  measurements  see  especially  The  Svedberg,  Nor.  Ac. 
Soc.  Sc.  Upsaliensis,  I.e.,  143. 

2  By  an  analogous  method,  M.  de  Broglie  has  measured  the  Brownian  movement 
in  gas  suspensions. 

3  M.  Seddig,  Sitz.  Marburger  Ges.  Nov.  1907;  Physik.  Zeitschr.  9,  465  (1908); 
Habilitationsschrift,  Frankfurt  a.  M.,  1909. 

4  H.  Siedentopf,  Zeitschr.  f.  wiss.  Mikrosk.  26,  407  (1909). 

13 


194 


SPECIAL  COLLOID-CHEMISTRY 


microscope,  to  the  right,  the  source  of  light,  as  an  arc  lamp.  As 
the  time  is  known,  the  average  velocity  of  the  particles  may  be 
determined  very  closely  by  measuring  the  plates  and  the  changes 
in  the  position  of  the  particles.  Most  of  the  paths  of  Brownian 
movement  reproduced  in  Figs.  34  to  37  were  obtained  in  this 
manner. 

H.  Siedentopf  uses  a  falling  photographic  plate.     In  this  wise 
he  obtains  curves  as  shown  in  Fig.  40,  which  correspond  to  those 


FIG.  39. — V.  Henri's  apparatus  for  photographing  Brownian  movement 
cinematographically. 

first  observed  by  Svedberg.  Svedberg  (I.e.)  has  more  recently 
used  a  photographic  method  in  which  the  position  changes  are 
registered  as  points  on  a  rotating  film.  For  details  his  original 
paper  must  be  consulted.  Finally  it  should  be  mentioned  that 
P.  Chaudesaigues1  and  later  J.  Perrin  in  the  extended  work 
already  referred  to,  also  calculated  the  velocity  by  following  the 
position  changes  of  the  particles  at  definite  intervals  with  the  aid 
of  a  drawing  apparatus. 

The  rotary  motion  which  many  particles  show  has  been  studied 
by  J.  Perrin.2     To  this  end  he  measured  the  rotary  movement  of 

1  P.  Chaudesaigues,  Compt.  rend.  149,  1044  (1908). 
*  J.  Perrin,  Compt.  rend.  149,  549  (1909). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


195 


small  excrescences,  such  as  air  bubbles,  eccentrically  attached  to 
the  larger  globules  of  a  mastic  hydrosol  in  unit  time. 

By  these  methods  that  most  characteristic  property  of  Brown- 
ian  movement,  namely  its  velocity,  can  be  accurately  measured 
and  the  influence  of  different  external  conditions  on  it  be 
studied. 

4.  Uniformity  of  Brownian  Movement. — A  law  of  fundamental 
importance  to  the  theory  of  Brownian  movement  has  been  stated 


FIG.  40. — Brownian  molecular  movement.     (According  to  H.  Siedentopf.)     Instan- 
taneous exposure  of  a  falling  plate.     Zeiss  dark  ground  condenser. 

by  Svedberg.1  The  amplitude  of  movement  is  directly  proportional 
to  the  period  of  vibration,  in  other  words,  to  the  time  required  by  a 
particle  to  complete  a  whole  backward  and  forward  vibration. 
If  this  is  represented  by  2.4,  while  by  2t  is  represented  the  period 
of  vibration,  the  relation  between  the  two  maybe  expressed  thus: 

A 

—  =  const. 

This  law  may  also  be  stated  thus:  The  velocity  of  Brownian 
movement  is  uniform.  As  Svedberg  (I.e.)  emphasizes,  this  is  of 
great  importance  because  it  compels  the  conclusion  that  the  forces 
of  Brownian  movement  cannot,  for  example,  be  elastic  in  type. 
The  period  of  elastic  vibrations,  as  of  those  of  the  pendulum,  is 
independent  of  the  amplitude  or  path  length. 
1  The  Svedberg,  Z.  f.  Elektroch.,  12,  853,  909  (1909). 


196 


SPECIAL  COLLOID-CHEMISTRY 


This  law  may  be  experimentally  tested  in  different  ways.  The 
path  of  a  vibrating  particle  may  be  divided  by  the  time.  If  the 
law  is  valid  the  quotient  must  be  constant.  Or  the  amplitude 
which  varies  with  the  temperature  and  the  time  of  vibration  may 
be  measured  at  different  temperatures.  The  most  convenient 
method,  perhaps,  is  to  compare  the  movements  of  equally  sized 
particles  in  liquids  of  different  viscosities  and  see  if  the  quotient 

^ —  is  constant,  for,  as  will  be  developed  more  fully  later  (page  197), 

their  velocity  is  markedly  dependent  on  the  viscosity  of  the  disper- 
sion means.  The  Svedberg  (I.e.,  1906)  has  used  this  last  method. 
The  following  table  shows  how  in  colloid  platinum  the  path 
length  divided  by  the  time  yields  a  constant  in  different  dis- 
persion means.  In  judging  the  results  the  rather  large  experi- 
mental error  must  be  kept  in  mind. 

TABLE  30. — PROPORTIONALITY  BETWEEN  THE  AMPLITUDE  AND  PERIOD  OF  VIBRATION 

IN  THE  BROWNIAN  MOVEMENT  OF  COLLOID  PLATINUM 

(According  to  The  Svedberg) 


Dispersion  means* 

Amplitude, 
2A  in  M 

Period  of  vibration, 
2t  in  sec. 

1 

Quotient 

2A 

—  =  const. 

Acetone  

6.2 

1 
0.032 

194(18°) 

Ethyl  acetate  ' 

3-9 

0.028 

I39d90) 

Amyl  acetate  

2.9 

0.026 

II2(l80) 

Water  , 

2.1 

0.013 

162(20°) 

Normal  propyl  alcohol  .... 

1 

1.3                                      0.009 

145(20°) 

*That  the  temperature  varied  between  18°  and  20°  must  be  kept  in  mind. 

5.  Influence  of  the  Specific  Surface  of  the  Particles. — Not 
until  a  certain  diameter  is  attained,  which  Wiener  (I.e.)  judged 
to  be  about  3-5;*,  do  dispersed  particles  begin  to  show  a  noticeable 
Brownian  movement.  The  rapidity  of  movement  increases  with 
decrease  in  the  size  of  the  particles  if  all  other  conditions  remain 
constant.  The  following  figures,  taken  from  F.  M.  Exner  (lc.}y 
illustrate  the  dependence  at  constant  temperature  (23°)  of  veloc- 
ity upon  size  in  the  case  of  gutta-percha  particles. 

TABLE  31. — DEPENDENCE  OF  BROWNIAN  MOVEMENT  ON  SIZE  OF  PARTICLES 

(According  to  F.  M.  Exner) 
Diameter  of  Velocity  of 

particles  in  n  particles  in  p  per  sec. 

1.3  2-7 

0.9  3-3 

0.4  3-8 


MECHANICAL   PROPERTIES   OF   COLLOID    SYSTEMS  1 97 

The  average  absolute  velocity  of  colloid  particles,  according  to 
The  Svedberg  (I.e.)  is  about 

cm. 

0.02  to  0.04  — 
^sec. 

ii 

or  200  to  400 

sec. 

in  other  words,  about  100  times  that  of  microscopic  coarsely  dis- 
persed particles.  There  exist,  of  course,  all  possible  transitions 
between  these  two  extremes.  It  would  be  of  interest  to  make  an 
extended  study  of  this  function  of  velocity  X  degree  of  dispersion. 

6.  Influence  of  Concentration  of  Dispersoid. — The  velocity 
depends  on  the  number  of  particles  in  the  unit  volume.     R.  Zsig- 
mondy1  found  "the  particles  to  influence  each  other  and  the  vigor 
of  their  movement  to  be  decreased  by  dilution  of  the  gold  par- 
ticles."    A  quantitative  study  of  this  important  relation  is  still 
lacking. 

7.  Influence  of  Viscosity  of  Dispersion  Means.— As  easily 
foreseen,  the  greater  the  viscosity,  the  slower  the  movement.     The 
Svedberg  (I.e.)  studied  this  relation  in  colloid  solutions,  finding  that 
the  relation  of  the  average  velocity  or  path  length  to  the  viscosity 
of  the  dispersion  medium  may  be  represented  by  a  hyperbolic 
function.     In  other  words,  the  relation 

At]  =  const., 

holds,  in  which  A  represents  the  amplitude  and  17  the  viscosity  of 
the  dispersion  means.  The  path  length  is  inversely  proportional 
to  the  viscosity  of  the  dispersion  means.  The  range  over  which  this 
law  is  valid  is  indicated  in  Table  32. 

TABLE  32. — DEPENDENCE  OF  PATH  LENGTH  OF  BROWNIAN  MOVEMENT  OF  COLLOID 

PLATINUM  PARTICLES  ON  VISCOSITY  OF  DISPERSION  MEANS 

(According  to  The  Svedberg) 


Dispersion  means 

Path  length 
2A  in  M 

Viscosity  in  c.g.s.    1           A 
unitsVo'                 2^,  =  const. 

Acetone.  . 

6  2 

32                                      IO    8 

Ethyl  acetate  
Amyl  acetate  
Water  

4.0 
3-o 

1                     2    2 

4.6                       18.4 

5.9                17.7 

IO    2                                      22    4 

Normal  propyl  alcohol  .  . 
^-Isobutyl  alcohol  

1.4 
1.2 

22.6                                    31.6 

39-3                          47-3 

1  R.  Zsigmondy,  Z.  Erkenntnis  d.  Kolloide,  in  (Jena,  1905). 


SPECIAL  COLLOID-CHEMISTRY 


As  can  be  seen,  the  product  2 A  i\  increases  when  the  higher 
viscosities  are  attained,  though  here  the  measurement  of  the 
path  length  is  very  difficult.  Table  33  brings  out  the  same 

TABLE  33.— DEPENDENCE  OF  PATH  LENGTH  OF  BROWNIAN  MOVEMENT  OF  COLLOID 

CALCIUM  PARTICLES  ON  VISCOSITY  OF  DISPERSION  MEANS 

(According  to  The  Svedberg) 


Dispersion  means 

Path  length 
2  A  in  n 

Viscosity  in  c.g.s. 
units  17.  io3 

2A.7, 

Ethyl  ether 

8  o~o  o 

2   4 

IO    2—21    6 

Ethyl  acetate  

4.  O—  <?   O 

47 

18  8-23  < 

Chloroform 

2    O—  3    O 

*  8 

II    6—17    A. 

Ethyl  alcohol  

O  .  <—  2  .  0 

12.7 

6  .  3  ^—2  <   4. 

Isobutyl  alcohol  

Immeasurably 

4.O   O 

p 

small 

facts  from  a  study  of  electrically  prepared  calcium  organosols. 
The  approximate  changes  in  path  length  are  again  shown. 

In  conclusion  it  should  be  pointed  out  that  this  law  of  inverse 
proportion  as  worked  out  experimentally  by  Svedberg  is  but  a 
corollary  of  the  law  of  Stokes,  formulated  in  1850,  which  covers 
the  relation  between  the  movement  of  small  bodies  and  the  forces 
acting  upon  them  (see  page  204). 

8.  Influence  of  Temperature. — Rising  temperature  accelerates 
Brownian  movement,  but,  it  must  be  remembered,  this  also  de- 
creases the  viscosity  of  the  dispersion  means.  Thus  gutta-percha 
particles,  o0/*  in  diameter,  have  an  average  velocity  of  3.2/4  per 
second  at  20°,  while  at  71°  they  have  one  of  5.1/1  per  second 
(F.  M.  Exner  I.e.).  M.  Seddig  (I.e.)  has  investigated  the  influence 
of  temperature  more  exactly.  He  finds  that  the  amplitude  or 
movement  of  the  particles,  after  a  definite  time,  is  dependent  on 
the  temperature,  according  to  the  formula: 

A  =  i 

in  which  A  is  the  amplitude,  T  the  absolute  temperature  and  77  the 
viscosity.1  The  values  as  determined  average  six  per  cent,  above 
the  calculated,  but  this  difference  is  plausibly  explained  as  due  to 
the  disturbing  influence  of  heat  upon  the  photographic  process 

1  Strictly  speaking,  Seddig  investigated  the  validity  of  the  above  formula  indi- 
rectly, in  that  he  determined  the  relation  of  the  amplitudes  to  each  other  at  different 
temperatures  and  not  the  change  in  their  absolute  value  with  the  temperature. 


MECHANICAL  PROPERTIES   OF   COLLOID    SYSTEMS 


199 


needed  to  record  the  movements.  This  explains  why  all  the  devia- 
tions occurred  in  the  same  direction.  For  details  Table  34  should 
be  studied. 

TABLE  34. — DEPENDENCE  OP  BROWNIAN  MOVEMENT  ON   TEMPERATURE 
(According  to  M.  Seddig)1 


Temperature  of 
observation 

Viscosity  of  dispersion 
means 

Relation  of  displace- 
ments 

Deviation 
in  per  cent. 

ti 

tt 

at  Ji 

at  It 

at  ti  and  tt 

Observed 

Calculated 

20.  o 

90.0 

O.OIOI 

0.0032 

2.07 

1.977 

+  4-7 

20.  o 

90.0     !    o.oioi          0.0032             2.08 

1.977 

+  5-2 

20.  o 

72.5          o.oioi 

0.00392    '        1.9 

1.743 

+  9-0 

17.0 

90.0          o.  01106 

0.0032              2.  20 

2.080 

+  5-8 

17.0 

72.5         0.01106 

0.00392            1.99 

1.833 

+  8.6 

17.0 

90.0        0.01106 

0.0032              2.18 

2.080 

+  4-8 

IO.O 

90.0 

0.01309 

0.0032              2.37 

2.290 

+  3-5 

17.0 

90.0 

o.  01106 

O.OO32                   2.21 

2.080 

+  6.3 

17.0 

90.0        0.01106 

0.0032                  2.19 

2.080 

+  5^9 

12.9 

90.0 

0.01258        0.0032 

2.32 

2.234 

+  3.9 

15.0 

90.0          0.0113 

0.0032 

2.22 

2.  HO 

+  5-2 

12.9 

90.0          0.01258        0.0032 

2-39 

2.234 

+  6.7 

5-5 

90.0        0.01494 

0.0032 

2.64 

2.463 

+  7-2 

The  observations  of  F.  M.  Exner  suffice  to  prove  the  validity  of 
this  function  as  J.  Perrin  (I.e.  1910,  page  269)  has  pointed  out. 
For  the  value  1.6,  which  expresses  the  ratio  of  the  path  lengths 

.—  at  a  temperature  increase  from  20°  to  71°,  almost  equals  1.7,  the 
square  root  of  the  expression 

342  X  o.oio 


T  1.112       293  X  0.004 

which  is  clearly  only  an  algebraic  transformation  of 'the  above 
equation.     If  this  is  given  the  form : 


it  is  seen  that  the  square  of  the  path  length  is  directly  proportional 
1  The  figures  used  in  this  table  were  collected  by  W.  Mecklenburg  (l.c.,  page  77). 


200  SPECIAL  COLLOID-CHEMISTRY 

to  the  ratio  of  the  absolute  temperature  and  the  viscosity.  If  the 
influence  of  the  latter  is  eliminated  by  using  Svedberg's  law  (Aij  = 
k),  the  result  is: 

AM  =  A.Arj  =  kiT;  Arj  =  const;    A  =  k-2.T, 

in  other  words,  the  path  length  is  directly  proportional  to  the 
absolute  temperature  when  the  viscosity  is  constant. 

9.  Influence  of  Added  Substances.— Even  the  earlier  authors 
knew  that  small  amounts  of  electrolytes  greatly  reduce  or  even 
stop  Brownian  movement.  As  a  rule,  this  cessation  is  closely 
connected  with  a  clumping  into  larger  complexes,  a  process  usually 
ending  in  a  precipitation  or  coagulation  of  the  system.  Hence  it 
has  been  assumed  [see  Svedberg,  (I.e.,  1907)]  that  the  addition  of 
electrolytes  retards  Brownian  movement  only  because  it  causes  an 
increase  in  the  size  of  the  vibrating  particles.  Though  this  view 
may  be  largely  correct,  recent  observations  have  shown  that 
retardation  of  Brownian  movement  may  occur  even  when  there  is 
no  clumping,  and  what  is  more  important,  it  may  even  be  ac- 
celerated on  adding  electrolytes.  Thus  V.  Henri  (I.e.)  found  the 
movement  of  the  latex  globules  in  caoutchouc  juice  to  be  reduced 
by  half  on  adding  N/io  NaOH,  and  to  one-ninth  the  original  rate 
when  N/32  HC1  was  added,  even  though  no  clumping  could  be 
detected.  While  the  path  length  normally  averaged  0.62^  per 
Ho  second,  it  was  reduced  to  0.3  i/z  in  the  alkaline  medium  and  to 
but  0.07/^1  in  the  acid.  The  normal  path  of  these  globules  is 
shown  in  Fig.  34.  The  path  in  an  alkaline  medium  is  shown 
in  Fig.  35;  that  in  an  acid  one  in  Fig.  36.  Lecoq1  observed  the 
addition  of  electrolytes  distinctly  to  increase  the  movement  of  col- 
loid arsenic,  but  unfortunately  he  gives  no  details  as  to  the  sub- 
stances and  concentrations  used. 

The  retardation  might  be  explained  by  assuming  that  the  added 
ions  are  absorbed  by  the  latex  globules  causing  an  enlargement  of 
the  particles  and  a  slowing  of  their  movement  through  the  hydrate 
envelopes  added  in  this  way.  But  this  explanation  does  not 
harmonize  with  Lecoq's  results,  who  found  the  rate  to  increase  on 
adding  electrolytes.  Perhaps  we  need  to  consider  other  factors, 
such  as  electrical  ones. 

The  retarding  influence  of  certain  non-electrolytes  such  as 
1  Lecoq,  Compt.  Rend.,  150,  700  (1910). 


MECHANICAL  PROPERTIES   OF   COLLOID   SYSTEMS  2OI 

urea  (Perrin,  I.e.}  on  Brownian  movement  can  easily  be  explained 
through  the  increase  in  viscosity  of  the  dispersion  means  which  they 
bring  about. 

10.  Influence  of  the  Electrical  Charge. — The  investigations  of 
The  Svedberg  and  J.  Perrin  (discussed  on  p.  190)  proved  con- 
clusively that  the  degree  of  movement  of  vibrating  particles  was 
independent  of  their  electrical  charges.  These  are  the  only 
investigations  available  on  this  point.  Their  repetition  and  ex- 
tension to  other  dispersed  particles  is  greatly  needed.  The  follow- 
ing theoretical  considerations  make  this  complete  independence 
appear  strange.  As  familiarly  known  from  the  study  of  gaseous 
ions,1  an  electrically  charged  particle  induces  in  its  surroundings 
an  electromagnetic  field  which  opposes  its  movement.  One 
would  therefore  assume,  if  any  effect  of  the  particles  upon  each 
other  were  excluded,  that  the  spontaneous  movement  would  de- 
crease as  the  electric  charge  increased  and  that  when  the  charge  is 
zero,  in  other  words,  at  the  iso-electric  point,  motion  would  be 
greatest.  As  a  matter  of  fact,  traces  of  electrolytes  when  ad- 
sorbed by  the  vibrating  particles  may  retard  their  movement 
through  changes  in  their  charges  in  either  a  positive  or  a  negative 
sense  and  it  is  therefore  not  impossible  that  the  phenomena  ob- 
served by  Henri  and  Lecoq  may  be  associated  with  such  charging 
and  discharging. 

Perhaps  future  investigators,  using  more  exact  methods  than 
could  Svedberg,  will  prove  the  fundamental  though  not  the  func- 
tional independence  of  Brownian  movement  of  the  size  of  the 
charge  of  the  particles. 

n.  Influence  of  Gravity  on  the  Distribution  of  Vibrating 
Particles. — J.  Perrin  (I.e.,  1910),  in  part  with  Chaudesaigues  and 
Dabrowski  has  studied  the  distribution  of  the  dispersed  particles 
in  fine  mastic  dispersoids  when  left  to  the  influence  of  gravity. 
The  problem  is  not  one  of  simple  sedimentation  in  the  ordinary 
sense  of  the  word,  for  these  systems  are  so  highly  disperse  (the 
particles  having  a  diameter  of  0.5-0.7/4)  that  a  settling  out  of  the 
disperse  phase  can  occur  only  after  a  very  long  time,  if  at  all. 
The  investigation  dealt  rather  with  the  stratification  of  particles 
showing  Brownian  movement.  Such  stratification  is  evidently 

1  See.  J.  J.  Thomson,  Conduction  of  Electricity  through  Gases,  Cambridge,  1903. 


2O2 


SPECIAL   COLLOID-CHEMISTRY 


the  result  of  a  force  (gravity)  acting  in  one  direction  on  the 
Brownian  movement.1  On  a  priori  grounds  we  would  expect  that 
the  greater  density2  or  "excess  weight"  possessed  by  some  of  the 
particles  would  add  to  the  previously  irregular  movement  a 
component  directed  downwards,  thus  changing  the  previously 
uniform  distribution  of  the  particles  into  an  uneven  one  in  which  a 
more  concentrated  layer  appears  at  the  bottom. 

Perrin's  investigations  were  not  carried  out  in  tall  cylinders  as 
one  might  be  inclined  to  expect,  but  in  small  microscopic  columns 
of  liquid  not  more  than  iooju  high.  The  arrangement  of  the 


Objective 


Cover     Glass 


j        Emulsion      ( 

\H 

Slide 


FIG.  41. — Arrangement  for  determining  the  distribution  of  particles  in  a  mastic 
dispersoid.     (According  to  /.  Perrin.) 


apparatus  is  shown  in  Fig.  41.  The  different  levels  in  the  liquid 
column  were  reached  and  studied  by  simply  raising  or  lowering 
the  objective.  The  particles  in  the  different  optical  sections 
were  counted  either  photographically  or  by  a  special  process3 
and  their  number  in  the  different  sections  compared.4  Figs.  42 

1  Analogous  stratifications  are  to  be  expected  under  the  influence  of  other  uni- 
directional forces,  as  electrical  or  magnetic. 

2  In  the  case  of  specifically  lighter  particles  we  would  expect  similar  differences  in 
distribution  to  result  in  the  formation  of  a  "scum." 

3  This  consisted  of  so  narrowing  the  field  of  vision  by  a  diaphragm  that  only  a  few 
(5-6)  particles  were  visible  at  a  time.    They  could  then  be  easily  counted.    The 
average  of  a  large  number  of  such  readings  (Perrin  made  thousands  in  some  cases) 
yields  with  sufficient  exactness,  the  number  of  particles  in  each  of  the  layers. 

4  Two  other  methods  which  seem  to  offer  advantages  and  which  might  be  used  are 
the  following:    Metallic  colloids  are  produced  in  molten  paraffin  and  poured  into 
heated  metallic  rings  or  tubes.     After  standing  some  time,  the  paraffin  is  solidified 
as  quickly  as  possible,  as  by  plunging  into  liquid  air.     Sections  are  then  cut  with  an 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


203 


and  43  are  photographs  obtained  by  Perrin  showing  the  distribu- 
tion of  gutta-percha  particles  when  such  methods  are  used. 


FIG.  42.  FIG.  43. 

FIG.  42. — Distribution  of  oscillating  gutta-percha  particles  under  the  influence 
of  gravity.  (According  to  /.  Perrin.}  (Diameter  of  field  0.6  n;  four  levels  10  /* 
apart.) 

FIG.  43. — Distribution  of  oscillating  mastic  particles  under  the  influence  of 
gravity.  (According  to  /.  Perrin.)  (Diameter  i  /x;  three  levels  12  /*  apart.) 

These  measurements  yielded  the  important  law:     The  con- 
centration of  the  disperse  phase  increases  in  geometric  progression 


ordinary  microtome  and  the  particles  in  each  counted  ultramicroscopically.  A  yet 
simpler  method  would  consist  in  filling  a  burette  with  a  suitable  dispersoid  and 
keeping  this  for  some  time  at  constant  temperature.  Different  layers  could  then  be 
carefully  drawn  off  and  their  content  of  the  disperse  phase  be  determined  either 
gravimetrically  or  by  titration. 


204 


SPECIAL   COLLOID-CHEMISTRY 


with  the  algebraic  decrease  in  the  height  of  the  level.  Symbolically 
expressed  this  would  be: 

2.303  log  £  =  g.h 

wherein  n0  is  the  concentration  (number  of  particles  per  unit 
volume)  in  the  initial  level  o,  nh  the  concentration  in  the  level  h,  g 
the  constant  of  gravity,  h  the  level  and  2.303  the  conversion 
factor  of  decadic  to  natural  logarithms.  The  following  table 
contains  a  compilation  of  several  series  of  such  experiments  made 
by  J.  Perrin  (I.e.,  1910). 

TABLE  35. — DEPENDENCE  OF  CONCENTRATION  ON  LEVEL  OF  LAYER  IN  SUSPENSIONS 
(According-  to  J.  Perrin) 


Outta-percha  Suspensions, 

Diameter  of  particles 

Mastic  Suspensions, 

Diameter  of  particles 

0.28/i 

0.424M 

about  GM 

about  1ft 

ght  of  level 

•*J 

culated  con- 
ntration 

ght  of  level 

>erved  con- 
ntration 

culated  con- 
ntration 

ght  of  level 

erved  con- 
centration 

culated  con- 
centration 

ght  of  level 

served  con- 
entration 

culated  con- 
ntration 

B 

S8 

6s 

A 

§8 

O8 

1 

S  " 

O 

3 

1 

Jc,  o 

O 

6s 

I00/i 

IOO 

IOO 

9°M 

12.0 

ii.  i       35M 

10          9.4 

24/i       305 

280 

75 

116 

119 

60 

22.6 

23.0 

25 

22 

21.0 

18 

530 

528 

So 

146 

142 

30 

47.0 

48.0 

15 

43 

45-0 

12 

940 

995 

25 

170 

169 

o 

IOO.O 

IOO.O 

5 

IOO        IOO.O 

6 

1880 

1880 

o 

200 

2OI 



1 

12.  Validity  of  Stokes'  Law  for  Highly  Dispersed  Particles.— 
G.  Stokes  formulated  a  law  in  1850  which  has  gradually  become 
famous.  It  expresses  the  relationship  between  the  velocity  of  small 
globules  and  the  forces  acting  upon  them,  such  as  gravity.  The 
law  may  be  expressed  thus: 

v  =  *D-d^r2 

9       v 

In  this  equation  v  is  the  velocity,  D  the  density  of  the  particle,  d 
the  density  of  the  liquid,  t\  the  viscosity,  K  the  constant  of  gravity 
and  r  the  radius  of  the  particle. 

•  In  a  given  dispersion  means,  at  constant  temperature,  etc.,  the 
velocity  of  a  particle  is  therefore  proportional  to  the  square  of  its 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  205 

radius.  Stokes  employed  this  law  in  calculating  the  speed  of  falling 
rain  drops.  What  interests  us  is  whether  this  law  also  holds  when 
the  degree  of  dispersion  is  very  high  as  in  dispersoids  showing 
Brownian  movements  (J.  Perrin,1  J.  Duclaux2). 

J.  Perrin  was  able  to  show  that  the  law  is  still  valid  for  particles 
with  a  radius  of  0.14  to  0.45/4,  in  other  words,  for  such  as  approach 
the  maximum  diameter  (iju)  of  colloid  particles.  He  measured  the 
sizes  of  the  particles  of  his  suspensions,  first,  by  calculating  their 
sedimentation  velocities  from  Stokes'  law,  second,  by  counting 
the  particles  in  a  known  volume  containing  a  known  amount  of 
disperse  phase,  and  thirdly,  by  a  micrometric  method  (for  the 
details  of  which  see  his  paper  of  1910).  The  three  methods  gave 
results  which  agreed  closely  with  each  other  as  evidenced  in 
Table  36. 

TABLE  36.— DETERMINATION  OF  STZES  OF  PARTICLES  BY  DIFFERENT  METHODS 

TO  TEST  APPLICABILITY  OF  STOKES'  LAW  TO  HIGHLY  DISPERSED  SYSTEMS 

(According  to  J.  Perrin) 

By  counting  !      According  to  Stokes'  law  Micrometrically 


0.46^1 

0-45M 

0.45-S/* 

0.30 

0.29 

0.30 

0  .  21  2 

O.  212 

O    Id. 

O  .  I  2 

w  •  A£f 

Whether  this  law  holds  for  systems  of  still  higher  degrees  of 
dispersion  has  not  yet  been  determined,  though  it  is  already  ap- 
plied with  remarkable  results,  not  only  to  the  theory  of  tnolecularly 
dispersed  solutions  (see  for  example  W.  R.  Bousfield3)  but  to  the 
migration  phenomena  of  gaseous  ions.4 

13.  Kinetic  Theory  of  Brownian  Movement. — As  mentioned 
before,  the  sources  of  energy  for  Brownian  movement  must  be 
sought  in  some  very  general  mechanical  forces  resident  within 
fluid  or  gaseous  dispersoids.  In  harmony  with  the  old  accepted 
and  widespread  kinetic  views  it  was  to  be  expected  that  Brownian 
movement  would  sooner  or  later  be  regarded  as  a  direct  result  of 
the  supposed  collisions  between  the  molecules  of  the  dispersion 

1  J.  Perrin,  Compt.  rend.,  146,  967  (1908);  Kolloidch.  Beih.,  I.e.,  1910. 

2  J.  Duclaux,  Compt.  rend.,  147,  131  (1908). 

3  W.  R.  Bousfield,  Z.  f.  phys.  Chem.,  53,  270  (1905). 

4  See  J.  J.  Thomson,  Conduction  of  Electricity  through  Gases,  Cambridge,  1903. 


206  SPECIAL   COLLOID- CHEMISTRY 

means.  As  a  matter  of  fact,  the  early  authors  (Chr.  Wiener,  G. 
Gouy,  etc.),  saw  in  this  its  only  possible  explanation.  Recently, 
A.  Einstein1  and  M.  von  Smoluchowski,2  in  some  exceedingly 
important  papers  on  molecular  physics  have  developed  by  some- 
what different  methods  a  theory  of  Brownian  movement  resulting 
in  two  almost  identical  formulae.  Their  fundamental  equation 
governing  the  kinetics  of  disperse  systems  reads: 


In  this  A  is  the  average  path  length  of  the  particle,  K  a  constant, 
R  the  gas  constant,  T  the  absolute  temperature,  N  the  number  of 
particles  in  a  gram  molecule  of  the  disperse  phase  (Avogadro's 
constant),  t  the  period  of  vibration,  t\  the  viscosity  of  the  dispersion 
means  and  r  the  radius  of  the  presumably  spherical  particle. 
The  formula  of  M.  von  Smoluchowski  differs  from  that  given 

above  only  in  having  the  factor  --  =  2.37  preceding  the  root  on 

the  right  side. 

The  derivation  of  the  formula  cannot  be  detailed  here.3  It 
will  only  be  shown  how  well  this  equation,  deduced  theoretically, 
agrees  with  the  experimental  results  of  The  Svedberg  and  J.  Perrin. 
It  should  be  emphasized  that  the  two  laws  formulated  by  Svedberg 
concerning  the  uniformity  of  Brownian  movement  and  its 
dependence  on  viscosity  were  discovered  before  he  had  any 
knowledge  of  the  Einstein-Smoluchowski  formula. 

Discussion  of  the  equation  leads  to  the  following  conclusions. 
If  we  assume  all  the  factors  in  the  equation  to  be  constant,  except 
the  path  length,  period  of  vibration  and  viscosity,  the  equation 
becomes 

A  =  KJ-  or  A2  =  K! •- 
\n  7? 

The  latter  form  states  that  not  the  path  length  but  its  square  is 
directly  proportional  to  the  period  of  vibration  and  inversely 
proportional  to  the  viscosity  of  the  dispersion  means.  Svedberg, 

1  A.  Einstein,  Ann.  d.  Physik  (4),  21,  17,  549  (1905);  (4),  19,  371  (1906);  Z.  f. 
Elektrochem.,  13,  41  (1907). 

2  M.  von  Smoluchowski,  Ann.  d.  Physik  (4),  21,  756  (1906). 

3  See  the  original  papers  as  well  as  the  excellent  pamphlet  of  W.  Mecklenburg,  Die 
experimentelle  Grundlegung  der  Atomistik,  Jena,  1910. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


207 


however,  found  (see  p.  197)  the  first  power  of  the  path  length 
to  be  proportional  to  the  period  of  vibration  and  inversely  pro- 
portional to  the  viscosity.  As  a  matter  of  fact,  the  above  equation 
can  be  separated  into  two  of  the  form: 


If  one  of  the  factors,  say  --,  is  constant,  as  demanded  by  the  kinetic 

gas  theory  (which  assumes  uniformity  of  average  velocity  of  the  gas 
molecules)  and  as  Svedberg  found  it  to  be  experimentally,  then  the 
other  factor  AT?  must  also  be  constant.  Conversely,  assuming  the 

validity  of  Stokes'  law,  AT/  becomes  a  constant,  and  therefore  — 

also.  The  formulas  deduced  from  the  kinetic  theory  therefore 
really  cover  the  case  in  which  both  Svedberg  laws  are  simultane- 
ously active. 

Experience  therefore  confirms  the  molecule-kinetic  deductions 
of  the  authors  named. 

With  this  equation  it  now  becomes  possible,  conversely,  to 
calculate  the  absolute  value  of  Brownian  movement  when  vis- 
cosity, size  of  particles,  etc.,  are  known.  Svedberg  (I.e.)  and  V. 
Henri  (i.e.,  1908)  have  done  this.  Their  calculated  and  observed 
results  do  not  agree  absolutely,  but  they  are  of  the  same  order  of 
magnitude  and  the  deviations  are  all  of  about  the  same  proportion. 
Undoubtedly  the  arbitrariness  or  inexactness  of  some  of  the  con- 
stants used  may  therefore  be  held  responsible.  Table  37  shows  the 
more  important  of  these  calculations.  ' 


TABLE  37. — CALCULATION  OF  PATH  LENGTH  OF  COLLOID  PLATINUM  PAR- 
TICLES EXHIBITING  BROWNIAN  MOVEMENT,  AFTER  THE  EINSTEIN- 

SMOLUCHOWSKI  FORMULA 
(According  to  The  Svedberg) 


Dispersion  means 

&7IO» 

(in  sec.) 

A  observed 
(in/i) 

A  calculated 
(inM) 

A  found 

A  calculated 

Acetone 

0.032 
0.028 
0.026 
0.013 

O.OOQ 

2-3 

4-6 

5-9 

10.  2 
22.6 

3-i 

2.0 

i-5 
1.  1 
0.7 

0.71 
0.44 
0.38 
0.20 
O.II 

4-4 
45 
4.0 

5-5 
6-4 

Ethyl  acetate  
Amyl  acetate  

Water 

n  Propyl  alcohol  

208  SPECIAL    COLLOID- CHEMISTRY 

With  due  allowance  for  the  large  experimental  error,  the  value 
of  the  rotational  movement  of  disperse  phases  also  agrees,  as 
J.  Perrin  (I.e.)  has  shown,  with  that  derived  from  the  formula  of 
A.  Einstein.  The  law  developed  by  J.  Perrin  governing  the 
changes  in  concentration  of  a  suspension  at  different  levels 
(as  discussed  in  §11)  has  also  been  deduced  from  considerations 
of  the  kinetics  of  gases.  Only  the  constants  of  the  formulas  are 
different  in  the  two  cases.  Thus  while  the  density  of  the  earth's 
atmosphere  does  not  decrease  by  half  until  a  height  of  about  6 
kilometers  is  attained,  the  concentration  of  the  dispersoids  in- 
vestigated by  Perrin  often  fell  off  this  amount  when  the  difference 
between  levels  was  only  about  lo/j,. 

It  should  also  be  pointed  out  that  the  constant  N  of  the  Ein- 
stein-Smoluchowski  equation,  in  other  words,  the  number  of 
particles  in  a  gram-molecule,  which  is  of  such  great  importance 
in  various  fields  in  physics  and  physical  chemistry,  can  be  calcu- 
lated in  different  ways.  The  values  thus  obtained  agree  sur- 
prisingly well  with  those  obtained  by  entirely  different  means. 
Indeed  it  seems  as  though  these  methods  as  applied  to  submo- 
lecular  dispersed  systems  yield  the  most  exact  figures  of  this  fun- 
damental value  new  obtainable.  As  this  constitutes  one  of  the 
brilliant  achievements  of  colloid  or  dispersoid  chemistry  the 
following  table  taken  from  J.  Perrin  (I.e.,  1910)  is  given  in  full. 

TABLE  38. — DETERMINATION  OF  THE  NUMBER  OF  PARTICLES  IN  A  GRAM-MOLECULE 
(AVOGADRO'S  CONSTANT  N)  BY  DIFFERENT  COLLOID-CHEMICAL  METHODS 

(According  to  J.  Perrin) 
Phenomenon  Studied  N.  io~22 

Average  of  volume  in  liquid  state >       45 

From  the  dielectric  force  of  gases <     200 

By  using  Van  der  Waal's  equation. 60 

.  From  distribution  of  a  uniform  suspension 70.5 

From  the  average  displacement  in  a  given  time 71.5 

movement 

From  the  average  rotation  in  a  given  time 65 

Diffusion  of  dissolved  substances 40-90 

Mobility  of  ions  in  water 60-150 

Radiance  of  the  sky 30-7150 


Viscosity 
of 


Direct  measurements 
of  atomic  charge 


Emissions  of 
a  corpuscles 


Of  droplets  condensed  upon  ions 60-90 

Of  ions  attached  to  dust  particles 64 


Total  charge  emitted 62 

Time  constant  of  radium 70.5 

Helium  produced  by  radium 71 

Energy  of  the  Infra-red  spectrum 60-80* 

1  See  Perrin  (1910)  for  details  regarding  other  phenomena. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  2 09 

These  brilliant  results  fill  one  with  admiration  for  the  remark- 
able fertility  of  the  Emstein-Smoluchowski  equation,  especially 
when  it  is  remembered  how  many  still  purely  hypothetical  factors 
enter  into  its  composition.  Nothing  better  illustrates  the  daring, 
we  might  say,  of  this  train  of  thought  than  the  remark  of  Perrin, 
to  whom,  with  Svedberg,  science  owes  most  in  this  field,  anent  the 
theorem  of  the  equality  of  the  distribution  of  energy  which  is  the 
nucleus  of  all  kinetic  deductions.  "The  word  theorem  should 
deceive  no  one,  for  it  is  full  of  hypotheses  as  is  almost  every  theory 
of  mathematical  physics."  It  is  safest,  perhaps,  to  hold  that  the 
future  will  preserve  but  a  part  of  our  present  kinetic  notions  to 
work  over  into  a  more  general,  less  supposititious  theory.  As  a 
matter  of  fact,  several  of  the  laws  governing  Brownian  movement 
may  be  deduced  even  without  recourse  to  kinetic  assumptions,  as 
for  example,  the  inverse  proportionality  of  velocity  to  viscosity, 
from  Stokes'  law.1  Possibly  this  purely  inductive  method  will 
some  day  discover  these  same  laws;  in  fact,  consideration  of  the 
methods  of  science  demands  it,  but  when  the  day  will  come  must 
remain  a  matter  of  opinion. 

14.  Determination  of  the  "Molecular  Weight"  of  Dispersed 
Particles  from  their  Brownian  Movement. — Since  N  can  be  cal- 
culated, the  so-called  "molecular  weight"  of  dispersed  particles 
may  also  be  determined  from  the  formula  of  Einstein  and  Smo- 
luchowski.  This  can  also  be  done  from  the  logarithmic  distribu- 
tion equation  governing  concentration  in  different  levels.  J.  Per- 
rin2 made  such  calculations  and  by  this  method  found  his  gutta-per- 
cha particles  to  have  a  molecular  weight  of  about  30,000,000,000. 
It  must  again  be  emphasized  that  these  values  cannot  be 
compared  with  the  molecular  weights  of  molecularly  dispersed 
particles.  In  the  former,  the  diameter  of  the  particles  (or  their 
volume)  is  under  discussion,  and  this  "molecular  weight"  becomes 
progressively  less  as  the  size  of  the  particles  decreases.  The 
normal  concept  of  molecular  weight  does  not  consider  the  size 
of  the  particles  as  at  all  variable,  but  deals  simply  with  that  single 

1  The  influence  of  electrical  energy  upon  Brownian  movement  as  postulated  on 
p.   201  cannot  be  deduced  from  kinetic  considerations,  but  is  an  inductive  con- 
clusion.    It  appears  in  the  Einstein-Smoluchowski  equation  as  a  factor  analogous 
to  the  viscosity  factor  since  the  velocity  would  be  approximately  inversely  propor- 
tional to  the  intensity  of  the  induced  field  of  force.     Judging  from  the  experi- 
mental results  of  Svedberg  and  Perrin  these  proportionality  constants  would,  in 
many  cases,  have  a  very  small  value. 

2  J.  Perrin,  Compt.  rend.,  147,  475  (1908). 

14 


2IO  SPECIAL    COLLOID-CHEMISTRY 

value  which  is  observed  at  the  maximum  degree  of  dispersion. 
This  obviously  constitutes  a  fundamental  distinction  between  the 
"molecular  weights'7  of  differently  dispersed  systems. 

§28.  Diffusibility  of  Colloids 

i.  General  Remarks. — When  one  pours  some  of  the  pure 
dispersion  means  upon  a  molecularly  dispersed  system,  the  mo- 
lecularly  dispersed  phase  wanders  over  into  the  pure  dispersion 
means  until  uniform  distribution  throughout  both  phases  is  at- 
tained. This  phenomenon  is  known  as  diffusion.  In  trying  to 
explain  what  has  happened  it  is  natural  to  think  of  the  influence  of 
Brownian  movement.  In  the  irregular,  particularly  in  the  forward, 
movements  of  small  particles,  as  observed,  for  example,  by  Zsig- 
mondy  in  colloid  solutions,  it  is  to  be  expected  that  an  accidental 
wandering  of  the  particles  over  into  the  pure  dispersion  means  must 
take  place.  But  such  accidental  migration  cannot  wholly  explain 
all  diffusion,  the  laws  of  which  A.  Fick  formulated  in  1855.  In  order 
that  Brownian  movement  may  lead  to  diffusion,  it  must  become 
directive  in  character  toward  the  pure  dispersion  means  or  toward 
the  "more  dilute"  parts  of  any  continuous  system. 

As  a  matter  of  fact,  the  existence  of  such  a  directive  movement 
in  diffusion  until  uniform  distribution  of  the  dispersed  phase  through- 
out the  whole  system  is  attained  can  be  foreseen,  when  the  relation 
between  degree  of  movement  and  concentration  of  dispersed  parti- 
cles is  called  to  mind.  As  noted  above  (p.  196),  R.  Zsigmondy 
observed  less  movement  in  dilute  systems  than  in  concentrated 
ones.  Because  of  this,  equilibrium  cannot  exist,  so  far  as  average 
velocity  of  particles  is  concerned,  in  a  system  consisting,  say, 
of  a  colloid  solution  covered  by  a  layer  of  the  pure  dispersion  means. 
In  places  of  greater  concentration,  the  particles  will  be  moving 
faster  than  in  those  of  a  lower  one.  The  sources  of  energy  for 
Brownian  movement,  whatever  they  be,  must  therefore  have  dif- 
ferent values  in  different  parts  of  the  system  at  the  beginning  of 
diffusion.  But  following  the  general  laws  of  energy,  equilibrium 
cannot  be  attained  in  a  closed  system  until  the  energy  intensities 
have  the  same  value  everywhere.1  We  need  but  call  to  mind  the 
electrostatic  charge  on  the  surface  of  a  metallic  sphere.  If  the 

1  Wilh.  Ostwald,  Lehrb.  d.  allgem.  Chem.,  2  AufL,  2,  35  (1903). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


211 


energy  intensities  in  a  "diffusion  field"  are  not  everywhere  the 
same  the  system  is  unstable  and  changes  must  occur  of  a  directive 
character  leading,  in  the  end,  to  an  equalization  of  the  intensities 
in  the  whole  system.  Thus,  if  only  a  local  charge  were  present 
on  the  metallic  sphere,  currents  would  emanate  from  this  to  all 
other  points  on  its  surface.  In  the  case  of  diffusion,  a  movement 
of  the  dispersed  particles  toward  regions  of  lower  concentration 
would  have  to  occur  until  the  average  velocity  of  the  particles  was 
the  same  in  all  portions  of  the  system.  The  average  absolute 
value  of  the  movement  would  therefore  become  progressively 
less  until  the  minimum  is  attained  at  the  end  of  diffusion.  It 
is  in  keeping  with  this  general  notion  that  in  the  molecular  dis- 
persoids  the  diffusion  coefficients  (dis- 
tances traversed  in  centimeters  per  day) 
are  greater  at  higher  concentrations  than 
at  lower  ones.1 

The  influence  of  concentration  on  the 
diffusion  of  a  colloid  has  not  yet  been 
studied.  The  experimental  difficulties 
besetting  such  a  study  will  become  clear 

in  the  following  paragraphs.  FIG.  44. — Apparatus  for 

-r\'ff     •  •  i  -i    the  study  of  diffusion  as  ar- 

Diffusion  experiments  have  a  special   ranged  by  Thomas  Graham. 

interest  in  colloid  chemistry  because  its 

very  foundations  were  built  upon  them  by  Th.  Graham  (1850- 

1862). 

2.  Experimental  Study  of  Diffusion  of  Colloids. — The  common 
method  of  determining  the  diffusion  coefficient  was  originated  by 
Th.  Graham.2  A  wide-necked  bottle  is  filled  with  the  solution  to 
be  investigated  and  placed  in  a  second  vessel;  the  pure  dispersion 
means  is  then  poured  with  special  care3  into  the  second  vessel 
until  it  covers  the  inner  bottle  to  a  depth  of  several  centimeters. 
Fig.  44  is  an  exact  copy  of  the  sketch  from  Graham's  original  work. 
After  a  given  time,  the  amount  of  dissolved  substance  which  has 
escaped  from  the  inner  vessel  is  determined.  The  relation  of  this 

1  The  complicated  diffusion  phenomena  observed  in  certain  ionic  dispersoids,  as 
in  hydrochloric  acid,  form  exceptions  to  the  general  rule  because  electrochemical 
processes  come  into  play.     SeeWilh.  Ostwald,  Lehrb.  d.  allg.  Chem.,  2  Aufl.,  i,  686 
(i903). 

2  Th.  Graham,  Philos.  Trans.,  1-46, 805-836  (1850);  483-494  (i85i),e  tc.;  Liebig's 
Ann.,  77,  56,  129  (1851);  121,  5,  29  (1862). 

3  To  prevent  mixing  of  the  two  liquids  at  the  critical  moment  Graham  used  a 
pointed  sponge  from  which  to  express  the  second  liquid. 


212  SPECIAL   COLLOID- CHEMISTRY 

to  the  time  (at  constant  diffusion  surface,  temperature,  etc.)  is  a 
measure  of  the  velocity  of  diffusion.  For  a  discussion  of  the  more 
modern  methods  of  using  Graham's  principle,  as  well  as  for  the 
methods  of  calculating  the  absolute  diffusion  coefficients  from  the 
experimental  data,  the  text-books  of  physics  and  physical  chemistry 
need  to  be  consulted.1 

It  is  difficult  in  Graham's  method  to  bring  the  two  liquids  into 
contact  with  each  other  without  disturbing  their  surfaces.  Slight 
differences  in  temperature,  vibrations,  etc.,  may,  moreover,  in- 
troduce great  experimental  errors.  But  Graham  already  knew  a 
remedy  for  this.  He  found  that  the  velocity  of  diffusion  was  not 
much  influenced  if  the  experiment  was  carried  out  in  a  not  too 
highly  concentrated  agar-agar,.  gelatin  or  starch  paste,  instead  of 
in  pure  water.  Thus,  when  he  placed  in  the  diffusion  cell  a  2  per 
cent,  agar  solution  containing  10  per  cent,  salt,  and  a  pure  agar 
solution  of  the  same  concentration  in  the  outer  vessel  and  allowed 
both  to  solidify,  he  found  after  15  to  16  days  that  the  latter  con- 
tained 9.992  grams  of  diffused  salt.  Normal  diffusion  into  pure 
water,  after  14  days  showed  9.999  grams,  all  other  conditions,  in- 
cluding temperature  (10°),  being  constant.  These  findings  have 
often  been  verified.  Thus  F.  Voightlander2  observed  0.72  per 
cent,  sulphuric  acid  to  diffuse  the  following  distances  into  agar 
jellies  of  different  concentrations  after  i  hour. 

Agar  jelly,  i  per  cent.  =  8.5  mm. 
2  per  cent.  =  7.8 
4  per  cent.  =  7.0 
The  amounts  that  diffused  were  as  follows: 

Into  agar  jelly,  i  per  cent.  =  1.08  mg.  S03 
2  per  cent.  =  i.io 
4  per  cent.  =  1.09 

The  absolute  values  for  NaCl  of  the  diffusion  coefficients, 

amounts  diffused  in  grams 

,  are  as  follows : 
days 

Agar  jelly,  i  per  cent.  =  1.04 

2  per  cent.  =  1.03 

3  per  cent.  =  1.03 

1  See,  for  example,  Wilh.  Ostwald,  Grundr.  d.  allg.  Chem.,  4  AufL,  194,  Leipzig, 
1909;  Wilh.  Ostwald-Luther-Drucker,  Hand  und  Hilfsbuch,  3  Aufl. 
1  F.  Voightlander,  Z.  f.  physik.  Chem.,  3,  329  (if 


MECHANICAL    PROPERTIES    OF    COLLOID    SYSTEMS  213 

G.  Hiifner1  and  others  obtained  similar  results.  But  it  should 
again  be  emphasized  that  diffusion  is  thus  independent  of  the 
presence  of  gels  only  when  these  are  there  in  low  concentrations. 
Marked  retardations  appear  at  higher  concentrations  as  even  H.  de 
Vries2  knew.  Diffusion  is  also  influenced,  of  course,  when  chem- 
ical or  colloid-chemical  changes,  as  precipitations,  are  produced 
in  the  gels  by  the  diffusing  substances. 

A  handy  arrangement  for  demonstrating  diffusion  l\as  already 
been  described  in  the  practical  introduction  on  p.  9.  Test  tubes 
are  half  filled  with  colloid  gels  and  the  diffusing  solution  poured 
upon  them.  Figs.  2,  45  and  46  illustrate  the  results. 

Disturbance  of  the  diffusion  surfaces  may  also  be  avoided  by 
stretching  over  the  inner  vessel  a  suitable  membrane  through 
which  the  dissolved  substances  pass  freely.  Hydrophane  plates 
(G.  Hiifner,  I.e.),  filter  papers  (S.  Exner,  see  below),  parchment 
papers  (The  Svedberg,  see  below),  etc.,  have  been  used  for  this  pur- 
pose. Or,  the  diffusing  substance  may  be  placed  directly  in  cells 
entirely  made  of  such  substances.  But  the  membranes  used  must 
be  completely  permeable  to  the  diffusing  substance  and  must  not 
affect  it,  as  through  adsorption,  etc.  §29  on  dialysis  should  be 
studied  in  this  connection. 

3.  Experimental  Facts  Regarding  Diffusion  of  Colloids.— It 
follows  from  the  relation  between  velocity  of  Brownian  movement 
and  size  of  particles  discussed  above  that  the  velocities  of  colloid 
particles  must  be  considerably  less  than  those  of  molecularly  or 
ionically  dispersed  ones.  The  compilation  in  Table  39  shows  this 
clearly;  additional  facts  regarding  diffusion  velocities  are  given 
below. 

As  is  clearly  evident,  the  diffusion  coefficients  of  typical  col- 
loids average  34  o  that  °f  the  slowly  diffusing  cane  sugar  and  only 
3/Loo  that  of  the  rapidly  diffusing  electrolytes  such  as  acids  and 
alkalies.  The  highly  dispersed  goldsol  of  The  Svedberg  which,  for 
a  colloid,  diffuses  exceptionally  fast,  takes  an  intermediate  posi- 
tion. It  should  be  remembered  that  the  particles  of  the  latter 
have  a  diameter  of  about  i/*/*;  in  other  words,  this  goldsol  is  OIL 
the  boundary  between  molecular  and  colloid  dispersoids. 

1  G.  Hiifner,  Z.  f.  physik.  Chem.,  27,  227  (1898). 

2  H.  de  Vries,  Fittica's  Jahresber.  d.  Chem.,  I,  144  (1884). 


214  SPECIAL    COLLOID-CHEMISTRY 

TABLE  39. — DIFFUSION  COEFFICIENTS  OF  DISPERSOIDS 


Molecular  and  ionic  dispersoids. 
Specific  area  >  6  X  IQ1 


Colloids. 
Specific  area  about  6  X  loUo  6  X  10* 


Nitric  acid  (Voightlander) . .  2 . 10  (20°) 

Sodium    chloride     (Voight- 
lander)     i .  04  (20°) 

Magnesium  chloride  (Voight- 
lander)  0.77  (20°) 

Copper  sulphate   (Landolt- 
Barnstein) 0.47  (17°) 

Urea  (Scheffer1) 0.81  (7.5°) 

Cane   sugar  (Graham- 
Stefan2) 0.31  (9°) 

Mannite  (Scheffer) 0.38  (10°) 


Gold  hydrosol  (The  Sved- 
berg6) 0.27(11.7°) 


Clupeinsulphate  (Herzog)  .  0.074  (18°) 

Pepsin  (Herzog3) 0.070  (18°) 

Rennin  (Herzog)  0.066  (18°) 

Egg-albumin  (Herzog). . . .  0.059  (18°) 
Albumin  (Graham-Stefan).  0.063  (13°) 
Caramel  (Graham-Stefan).  0.047  (10°) 

Ovomucoid  (Herzog) 0.044  (18°) 

Emulsin  (Herzog) 0.036  (18°) 

Invertin  (Herzog) 0.033  (18°) 

Diphtheria-toxin     (Arrhe- 

nius  and  Madsen4) 0.014  (12°) 

Diphtheria-antitoxin    (Ar- 

rhenius  and  Madsen) 0.0015  (12°) 

Tetanolysin  (Arrhenius  and 

Madsen) 0.037  (12°) 

Antitetanolysin  (Arrhenius 

and  Madsen) 0.0021  (12°) 


Figs.  45  and  46  illustrate  quantitatively  the  diffusion  velocities 
of  various  dispersoids.  They  show  what  has  happened  after 
about  3  day's  diffusion  into  solid  1.5  per  cent,  agar  at  20°. 
In  Fig.  46  the  supernatant  liquids  out  of  which  diffusion  has 
occurred  have  been  poured  off  so  that  the  diffusion  phenomena  may 
show  up  more  clearly.  To  the  left  in  this  figure  are  found  molecular 
dispersoids,  to  the  right,  typical  colloids.  The  tubes  are  arranged, 
from  left  to  right,  according  to  the  lengths  of  the  diffusion  paths.6 
The  picric  acid,  cobalt  nitrate  and  eosin  of  tubes  i,  2,  and  3  have 
wandered  almost  to  the  bottom  of  the  agar  column;  benzo-pur- 
purin  and  congo  red  on  the  extreme  right  have  scarcely  moved. 
The  dyes  lying  between  these,  show  intermediate  degrees  of  diffus- 
ibility.7  Fig.  45  shows  the  results,  after  8  days,  of  experiments 
on  the  diffusion  of  typical  colloids  (hydrosols  of  silver,  gold,  anti- 
mony sulphide,  arsenic  sulphide  and  iron  hydroxide).  The  sharp- 

1  G.  Scheffer,  Z.  f.  physik.  Chem.,  2,  390  (1888). 

2  Graham-Stefan,  Sitz.  Ber.  Ak.  Wien,  77,  II,  161  (1879). 

3R.  O.  Herzog  (and  H.  Kasarnowski)  Koll.  Zeitschr.,  2,  i  (1907);  3,  83  (1908); 
Bioch.  Zeitschr.,  n,  172  (1908). 

4  S.  Arrhenius  and  Th.  Madsen,  Immunochemie,  16,  Leipzig,  1907. 

'The  Svedberg  Z.  f.  physik.  Chem.,  67,  107  (1909) 

•  The  gradation  is  not  as  clearly  shown  in  the  photograph  as  it  actually  appears 
since  the  different  (mostly  o.i  per  cent.)  solutions  have  different  colors.  A  photo- 
graphic plate  does  not  bring  this  out 

7  Regarding  the  diffusibility  of  dyes  see  L.  Vignon,  Compt.  rend.,  150,  690  (1910). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


215 


ness  of  the  diffusion  line  between  the  diffusing  substance  and  the 
gel  should  be  noted.  It  is  sharp  in  the  case  of  typical  colloids; 
but  illy  marked  in  that  of  the  molecular  dispersoids  and  systems  of 
intermediate  degrees  of  dispersion. 

4.  Influence  of  Degree  of  Dispersion  on  Diffusion  Velocity. — 
The  diffusibility  of  a  disperse  phase  is  intimately  connected  with 
its  degree  of  dispersion  as  shown  in  Table  39.  Among  molecularly 


FIG.  45. — Diffusion  of  colloids  into  2  per  cent,  agar-agar  at  the  end  of  a  week, 
i,  Gold  hydrosol;  2,  silver  solution  (Credi}\  3,  antimony  sulphide  solution;  4, 
arsenic  trisulphide  solution;  5,  iron  hydroxide  solution. 


dispersed  substances,  ions  or  electrolytes  migrate  most  rapidly. 
Substances  of  higher  molecular  weight,  or,  more  correctly,  of 
greater  atomic  aggregation,  follow.  Last  in  the  list,  stand  the 
colloids.  This  dependence  of  diffusion  velocity  on  the  size  of  the 
particles  is  of  great  interest.  Of  special  importance  is  the  possi- 
bility of  procuring  one  and  the  same  substance  in  different  degrees 
of  dispersion  and  therefore  possessed  of  different  degrees  of 


2l6 


SPECIAL    COLLOID-CHEMISTRY 


diffusibility.  Thus  S.  E.  Linder  and  H.  Picton1  were  able  to  pre- 
pare the  following  four  systems  of  arsenic  trisulphide  in  water: 

c*As2S3;  particles  microscopically  visible,  non-diffusible  (coarse 
suspensions) , 

j8As2S3;  microscopically  homogeneous,  non-diffusible, 

7As2S3;  diffusible,  but  unfilterable  through  porcelain  cups, 

5As2S3;   diffusible  and  filterable. 

After  Wo.  Ostwald2  had  repeatedly  emphasized  the  great  theo- 
retical interest  attaching  itself  to  a  systematic  and  quantitative 


FIG.  46. — Diffusion  into  a  2  per  cent,  agar-agar  at  the  end  of  three  days. 
i,  Picric  acid;  2,  cobalt  nitrate;  3,  0;i  per  cent,  eosin;  4,  o.i  per  cent,  ponceau 
R.  R.  R.;  5,  o.i  per  cent,  new  fuchsin  O;  6,  o.i  per  cent,  vesuvin;  7,  o.i  per  cent, 
safranin  G.;  8,  o.i  per  cent,  benzopurpurin;  9,  o.i  per  cent.  Congo  red. 

investigation  of  the  relations  between  diffusibility  (and  other 
properties)  and  degree  of  dispersion,  The  Svedberg,3  in  the  prose- 
cution of  his  experimental  study  of  the  Einstein-Smoluchowski 
formula  (see  above),  attacked  the  problem.  He  determined  the 
diffusion  velocities  of  different  gold  hydrosols  by  pouring  these 
into  parchment  cells  having  different  porosities.  His  results  are 
given  in  Table  40: 

1  S.  E.  Linder  and  K.  Picton,  Trans.  Chem.  Soc.  Lend.,  61, 114,  137,  148  (1892); 
67,  63  (1895);  71,  568  (1897);  87,  1906  (1905). 

2  See,  for  example,  Wo.  Ostwald,  Koll.-Zeitschr.,  I,  298  (1907). 

3  The  Svedberg,  Z.  f.  physik.  Chem.,  67,  105  (1909). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  217 

TABLE  40. — DEPENDENCE  OF  DIFFUSION  VELOCITY  ON  SIZE  OF  PARTICLES  IN 

GOLDSOLS 
(According  to  The  Svedberg) 


Size  of  particles  in  MM 

Concentration  of  in-          Concentration  of 
ner  liquid  in                    outer  liquid  in 
normality                         normality 

Relation  of  con- 
centrations to 
each  other 

14 
20-30 

I  .  5      io-4 
1.5      io-4 

1.5      io-6 
1.3     io-7 

IOO 

1  200 

The  gold  content  was  determined  colorimetrically.  The 
reciprocal  values  of  the  concentration  relations  are  measures  of  the 
diffusion  velocity.  In  other  words,  D\  =  k\  .  Koo>  when  D  is  the 
diffusion  coefficient  and  ki  the  proportionality  constant.  Similarly, 
D2  =  ^2^200-  When  the  ratio  of  these  diffusion  coefficients  is 
compared  with  the  size  of  the  particles  (taking  the  latter  to  average 

2.5  ,  100 

respectively  2.5  and  25/4)  we  observe,  since  —  ^-^  ------  that 

the  diffusion  velocity  is  approximately  inversely  proportional  to  the 
size  of  the  particles,  or  D.r  =  constant. 

True  it  is,  that  we  are  basing  these  conclusions  on  studies  in- 
volving but  two  degrees  of  dispersion.  An  investigation  cover- 
ing a  wider  range  would  be  of  great  interest. 

Finally,  it  should  be  mentioned  that  S.  Exner1  found  coarse 
suspensions,  such  as  clay  silt,  to  show  a  distinct  though  slow  diffu- 
sion. But  whether  pure  dispersions  made  up  of  particles  larger 
than  5/j  and  therefore  free  from  Brownian  movements  are  really 
capable  of  true  diffusion  appears  doubtful  (see  below,  p.  219). 

5.  Theory  of  Colloid  Diffusion.  —  The  close  relation  between 
Brownian  movement  and  diffusion  was  mentioned  at  the  beginning 
of  this  paragraph.  It  seems  natural,  therefore,  that  the  moleculo- 
kinetic  considerations  of  A.  Einstein  and  M.  von  Smoluchowski,2 
which  proved  so  fruitful  in  the  mathematical  discussion  of  Brown- 
ian movement,  should  lead  to  similarly  important  results  when  ap- 
plied to  diffusion.  For  example,  the  inverse  proportion  between 
size  of  particles  and  diffusion  velocity  is  deducible  from  the  equa- 
tions of  Einstein  and  von  Smoluchowski.  For  the  diffusion  co- 
efficient they  developed  the  equation: 


N 

1  S.  Exner,  Sitz.  Ak.  Wiss.,  Wien,  56,  116  (1867). 

2  According  to  von  Smoluchowski  the  right  side  of  the  equation  contains  the 
factor  2.03. 


2l8  SPECIAL   COLLOID-  CHEMISTRY 

The  symbols  have  again  the  meaning  indicated  on  p.  206,  r  repre- 
senting the  radius  of  the  particles.  If  dispersion  means,  tempera- 
ture, internal  friction,  etc.,  are  constant,  the  diffusion  coefficients 
of  two  dispersed  phases  bear  the  following  relation  to  each  other: 

D1  _  rt 
D2  ~  ri' 

in  other  words,  they  correspond  to  Svedberg's  experimental  find 
ings. 

This  relation  has  much  in  common  with  the  equation  which 
expresses  the  connection  between  the  diffusion  of  molecular  dis- 
persoids  and  their  molecular  weight.  The  relation: 


D  •  V  m  = 


constant 


has  been  established  by  S.  Exner,  for  gases,  and  by  L.  L.  Oholm1 
for  (theoretically)  infinitely  dilute  solutions  of  non-electrolytes. 
In  this  equation  m  is  the  molecular  weight.  If  the  square  root  of 
the  molecular  weight  is  made  equal  to  the  radius  of  the  particles, 
this  equation  changes  into  that  governing  the  diffusion  of  goldsols. 

Conversely,  with  the  laws  of  Exner-Oholm  and  Einstein- 
Smoluchowski,  we  may  calculate  the  size  of  the  particles  as  well  as 
their  molecular  weight.  In  this  way  R.  0.  Herzog  (I.e.)  found  an 
approximate  agreement  between  the  "  molecular  weights"  of  ov- 
albumin,  hemoglobin,  etc.,  thus  calculated:  and  the  figures  obtained 
by  other  methods.  For  the  values  for  toxins,  etc.,  as  calculated 
by  Sv.  Arrhenius  and  Th.  Madsen  (I.e.),  control  measurements  are 
not  yet  available.  The  same  objections  may  be  raised  against  all 
these  calculations  which  were  raised  in  discussing  the  determi- 
nation of  the  molecular  weight  of  colloid  systems  by  freezing 
point,  boiling  point  and  vapor  pressure  methods. 

The  calculations  by  R.  O.  Herzog  and  The  Svedberg  (I.e.)  of 
the  size  of  the  particles  by  the  formula  of  Einstein-Smoluchowski 
are  less  open  to  objection.  Herzog,  on  this  basis,  calculated  the 
size  of  the  particles  of  ovalbumin  to  be  2.  86^-  This  figure  about 
corresponds  to  the  higher  dispersion  values  obtaining  within  col- 
loid systems,  and  therefore  agrees  well  with  the  general  fact  that 
the  colloid  properties  of  the  albumins  place  them  near  the  mo- 

XL.  L.  Oholm,  Z.  f.  physik.  Chem.,  70,  378  (1910);  this  also  includes  the  earlier 
literature. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  2IQ 

lecularly  dispersed  systems.  Svedberg  calculated,  in  a  reverse 
manner,  the  size  of  the  particles  of  the  highly  dispersed  gold  solu- 
tion of  R.  Zsigmondy,  the  particles  of  which,  according  to  Zsig- 
mondy,  had  a  diameter  of  i  to  4w.  He  obtained,  by  Einstein's 
formula,  o. 94/^/1,  and  by  Smoluchowski's  2.16^,  obviously  a  very 
good  agreement.  The  calculated  diameter  of  the  particles  of 
molecularly  dispersed  systems  also  agrees  well  with  the  values 
obtained  by  other  methods. 

6.  Effect  of  Added  Substances  on  Colloid  Diffusion.  Spurious 
Diffusion  of  Colloids. — The  effects  upon  diffusion  of  adding  differ- 
ent substances  are  so  complicated,  even  in  molecular  dispersoids, 
that  general  laws  governing  them  have  not  been  formulated.1  It 
is  to  be  expected  that  these  relations  will  be  still  more  complicated 
when  phases  having  different  degrees  of  dispersion  are  mixed. 
The  more  important  phenomena  observed  when  colloid  systems  are 
mixed  with  molecularly  dispersed  ones  are  the  following: 

The  effect  of  electrolytes  on  the  diffusion  velocity  of  colloids 
may  be  discussed  under  two  headings — the  electrolyte  may  be 
added  to  the  diffusing  substance,  or  the  diffusion  of  the  colloid  may 
be  permitted  to  occur  into  the  solution  of  an  electrolyte.  In  either 
case,  different  results  may  be  expected,  depending  on  whether  the 
electrolyte  does  not  affect  the  degree  of  dispersion  of  the  colloid 
(which  is  exceptional)  or  whether  it  increases  or  decreases  it.  Both 
an  increase  and  a  decrease  in  the  degree  of  dispersion  on  adding 
substances  from  without  have  been  described  in  the  literature. 
An  illustration  of  the  latter  is  found  in  the  common  and  well-known 
effects  of  electrolytes  on  colloids  (aggregation,  coagulation);  an 
illustration  of  the  former  in  the  phenomena  of  peptization. 

The  inhibiting  e/ect  of  added  substances  on  diffusion  has  been 
studied  by  E.  von  Regeczy.2  He  found  pure  albumin  when  placed 
in  parchment-paper  tubes  to  diffuse  out  of  these  in  the  course  of 
12  hours  in  sufficient  amount  to  impart  a  decided  albumin  re- 
action to  the  outer  liquid.  But  when  some  solid  NaCl  was  pre- 
viously added  to  the  albumin,  no  trace  came  out.  S.  E.  Linder 
and  H.  Picton  (I.e.,  1905)  noted  a  similar  behavior  in  an  inorganic 
colloid,  arsenic  trisulphide.  They  allowed  a  highly  dispersed 
arsenic  trisulphidesol  to  diffuse,  on  the  one  hand,  into  water,  on 

1  See,  for  example,  Wilh.  Ostwald,  Lehrb.  d.  allg.  Chem.,  2  AufL,  674,  Leipzig,  1903. 

2  E.  von  Reg6czy,  Pfluger's  Arch.,  34,  431  (1884). 


22O  SPECIAL    COLLOID-CHEMISTRY 

the  other,  into  an  NH4C1  solution,  so  dilute  that  it  caused  no  visi- 
ble coagulation.     Their  results  are  given  in  the  following  table: 

TABLE  41.— DIFFUSION  OF  As2S3  SOL  INTO  PURE  WATER  AND  INTO  NH4C1 

SOLUTION 
(According  to  S.  E.  Linder  and  H.  Picton) 


Time 

Diffused  amounts  in  per  cent,  of  the  inner  fluid 

Into  pure  water 

Into  NH4C1 

24  hours 
48 
72 
96 

10  per  cent. 
14 

i  per  cent. 
3 

23 

An  antimony  sulphidesol  gave  similar  results  when  permitted 
to  diffuse  into  water  and  into  a  solution  of  tartar  emetic. 

An  example  of  how  the  addition  of  an  electrolyte  may  favor 
diffusion  of  a  colloid  is  found  in  Th.  Graham's  paper  (I.e.). 
He  observed  egg  albumin,  which  in  its  natural  state  is  slightly 
alkaline  and  diffuses  but  slowly,  to  diffuse  more  rapidly  if  it  is 
carefully  neutralized  with  acetic  acid.  While  after  a  week  but 
0.63  gm.  of  native  (alkaline)  albumin  diffused  out,  0.94  gm.,  in 
other  words,  30  per  cent,  more,  came  out  when  the  albumin  was 
neutralized.  The  neutralization  increases  the  degree  of  dispersion, 
as  proved  by  the  observations  of  Wo.  Pauli  and  others  (see  p. 
169),  who  found  neutral  albumin  to  increase  the  viscosity  of  water 
less  than  that  to  which  an  acid  had  been  added. 

H.  Picton  (I.e.,  1892)  made  similar  observations  on  suspen- 
soids  of  arsenic  trisulphide.  He  found  this  to  diffuse  rapidly 
when  still  contaminated  with  the  tartar  emetic  from  which 
it  was  prepared.  Whether  the  electrolyte  serves  to  increase  the 
degree  of  dispersion  in  this  case  remains  a  matter  of  question, 
though  such  an  influence  on  suspensoids  has  been  observed.  It 
is  more  reasonable  to  assume  that  the  electrolytes  in  their  rapid 
diffusion  simply  drag  the  colloid  particles  along  with  them,  a 
view  held  by  H.  Picton  himself;  or  that  the  movements  of  the 
liquid,  caused  by  the  diffusion  of  the  electrolytes,  set  up  currents 
which  bring  about  the  observed  results. 

The  interesting  experiments  of  W.  R.  Whitney  and  J.  Blake1 

1  W.  R.  Whitney  and  J.  Blake,  Journ.  Amer.  Chem.  Soc.,  26,  1339  (1904). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  221 

on  the  great  velocity  of  diffusion  of  goldsols,  produced  by 
reducing  ether  solutions  of  gold  chloride  by  means  of  acetylene, 
must,  no  doubt,  be  similarly  explained.  When  they  concentrated 
their  colloid  gold  at  the  lower  end  of  a  vertically  placed  cylinder 
by  electrophoresis  and  then  carefully  poured  pure  water 
upon  it,  they  observed  an  unusually  rapid  and  spontaneous 
upward  movement  of  the  gold  which  increased  with  the  increase 
in  the  concentration  of  the  gold.  The  observed  velocities  varied 
between  o.oi  cm.  and  0.24  cm.  per  hour.  When  it  is  recalled  that 
F.  Voightlander  (p.  212)  found  the  rapidly  diffusing  sulphuric 
acid  to  cover  only  0.85  cm.  per  hour  in  i  per  cent,  agar  while  the 
finest  goldsols  of  The  Svedberg  have  a  diffusion  coefficient  of  only 
0.27  (as  compared  with  one  of  2.0  for  sulphuric  acid  on  the  same 
scale),  it  becomes  impossible  to  believe  that  the  experiments  of 
Whitney  and  Blake  deal  with  true  diffusion  of  a  colloid  phase. 
The  diffusion  movements  of  the  molecular  dispersoids  present 
in  their  preparations  may  have  led  to  the  high  (apparent)  diffusion 
of  the  colloid  particles,  as  in  the  experiments  of  H.  Picton.  More 
probably  still,  the  gold  particles  became  loaded  with  gas  through 
the  electrical  treatment  to  which  the  gold  was  subjected  and  this 
then  led  to  their  rapid  rise.  Suitable  experiments  could  easily 
be  arranged  to  test  the  validity  of  such  an  explanation. 

The  favorable  effect  of  electrolytes  upon  the  diffusion  of  col- 
loids has  again  been  observed  when  they  are  permitted  to  diffuse 
into  solutions  of  electrolytes.  Thus  von  Wittich1  found,  as  far 
back  as  1856,  that  albumin  diffuses  more  easily  into  a  salt  solution 
than  into  pure  water.  Within  certain  limits,  the  diffusion  is  the 
more  rapid  the  greater  the  concentration  of  the  salt.  E.  von  Regeczy 
(/.c.),  M.  Oker-Blom2  and  others  have  since  studied  this  phe- 
nomenon. The  paper  of  M.  Oker-Blom  is  the  source  of 
Table  42. 

It  is  readily  apparent  that  the  amounts  of  diffused  albumin  in- 
crease with  increase  in  the  concentrations  of  NaCl,  but  in  the  in- 
termediate concentrations,  from  0.56  to  1.30  per  cent.,  a  region  of 
minimum  diffusion  is  observed.  What  follows  will  show  that  this 
need  by  no  means  be  due  to  experimental  error. 

To  explain  these  phenomena,3  we  need  but  remember  that 

1  von  Wittich,  J.  Muller's  Arch.  f.  PhysioL,  286  (1856). 

2  M.  Oker-Blom,  Skandinav.  Arch.  f.  PhysioL,  20,  102  (1904.) 

3  Wo.  Pauli,  Koll.-Zeitschr.,  3,  n  (1908). 


222  SPECIAL   COLLOID-CHEMISTRY 

albumin  solutions  are  more  strongly  hydrated,  in  other  words, 
swell  more  in  many  salt  solutions  than  in  pure  water.  We  may 
assume  that  in  this  process  the  free,  dispersed  albumin  particles 
wander  into  the  strongly  hydrating  dispersion  means  just  as  the 
liquid  wanders  into  the  solid  colloid  to  make  it  "swell."  A  suffi- 
ciently marked  hydration  of  the  dispersed  particles  must  separate 
them  from  one  another. 

TABLE  42. — DIFFUSION  OF  SERUM  ALBUMIN  INTO  NaCl  SOLUTIONS 
(According  to  M.  Oker-Blom) 


Concentration  of  NaCl  in  the  outer  liquid 


Amount  of  albumin,  in  grams,  diffused 
after  24  hours 


about 

0 

o  .  28  per  cent. 
0.56 

0.053 
0.053 
0.052 

0.74 

0.052 

0-93 

0.050 

1.30 
1.48 
1.86 

0.052 
0.058 
0.060 

2.38 

0.079 

At  the  present  time,  we  can  only  guess  at  what  must  be  the 
influence  of  several  colloids  upon  each  other  when  they  are  mixed, 
and  how  they  must  affect  each  other's  diffusion  velocity. 

The  influence  of  concentration  and  of  temperature  on  the  diffu- 
sion of  colloids  has  not  yet  been  studied.  Judging  from  the  find- 
ings of  Th.  Graham  (7.C.),  the  rate  of  increase  in  diffusion  velocity 
of  egg  albumin  with  the  temperature  is  about  as  great  as  that  of 
molecularly  dispersed  systems  under  the  same  circumstances,  but 
exact  figures  on  the  subject  are  still  wanting. 

§29.  Dialysis  of  Colloid  Systems 

I.  General  Remarks. — The  impeding  effect  of  concentrated 
gels  or  membranes  upon  free  diffusion  was  touched  upon  above. 
While  ordinary  electrolytes  pass  through  parchment-paper  mem- 
branes almost  as  rapidly  as  though  they  were  not  there,  albumin 
and  gum  arabic  cannot  penetrate  them.  Th.  Graham,  who  first 
investigated  this  phenomenon,  called  it  dialysis  (1861).  He  noted 
that  all  substances  which,  when  allowed  to  diffuse  in  the  open,  do 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  223 

so  only  slowly  or  not  at  all  are  also  restrained  by  parchment  mem- 
branes. On  the  other  hand,  those  which  diffuse  rapidly  are  not 
markedly  checked  in  their  movement  through  the  presence  of 
membranes.  This  difference  in  behavior  of  "  dissolved  "  substances 
toward  parchment  paper  formed  the  basis  of  the  whole  concept  of 
the  colloids.  Substances  which  do  not  dialyze  (or  pass  through 
parchment  paper)  Graham  called  colloids,  those  which  do,  crys- 
talloids. The  latter  systems  are  today  known  as  "molecular 
dispersoids." 

One  can  readily  accomplish  a  separation  of  the  different  classes 
of  dispersed  systems  by  dialysis.  As  a  matter  of  fact,  Graham 
called  his  fundamental  work  "Liquid  Diffusion  Applied  to  Analysis." 
By  using  a  constant  type  of  membrane,  systems  of  unknown  degrees 
of  dispersion  may  be  classified  into  such  as  dialyze  and  such  as  do 
not  (see  the  practical  introduction).  When,  by  any  method  what- 
soever, coarsely  dispersed  systems  have  been  excluded,  dialysis 
offers  a  convenient  method  of  distinguishing  between  the  colloid 
and  molecularly  dispersed  systems. 

It  must  be  emphasized  that  comparable  results  may  be  ob- 
tained only  by  use  of  one  and  the  same  kind  of  membrane.  The 
precipitation  membranes  of  copper  ferrocyanide  and  tannic  acid- 
protein,  for  example,  are  impermeable  even  to  many  molecular 
dispersoids  and  may,  therefore,  give  rise  to  the  phenomena  of  os- 
motic pressure  (see  the  following  paragraphs). 

2.  Methods  of  Dialysis. — Parchment  tubes,  parchment  dif- 
fusion capsules,  reed  tubes,  fish  bladders,  urinary  bladders,  egg 
membranes  and  amniotic  membranes  are  most  used  in  the 
dialysis  of  colloids.1  Membranes  of  collodion,  as  first  used  in  col- 
loid studies  by  G.  Malfitano,2  are  especially  convenient  in  many 
respects.  Their  preparation  is  discussed  in  the  practical  introduc- 
tion (p.  10).  Several  forms  of  dialyzers  were  illustrated  on  page 
ii.3  Because  of  their  historical  interest,  Figs.  47,  48  and  49  are 
introduced,  which  are  copies  of  the  two  types  of  apparatus  which 
Graham  used  in  the  great  work  upon  which  colloid  chemistry  is 
built. 

1  A  detailed  discussion  of  dialysis  and  its  methods  may  be  found  in  R.  P.  von 
Calcar,  Dialyse,  Eiweisschemie  und  Immunitat,  Leipzig-Leiden,  1908. 

2  G.  Malfitano,  Compt.  rend.,  139,  1221  (1904). 

3  For  a  new  form  see  R.  Zsigmondy  and  R.  Heyer,  Z.  f.  anorg.  Chem.,  68,  916 
(1910). 


224 


SPECIAL   COLLOID-CHEMISTRY 


In  dialyzing  non-aqueous  liquids,  the  effect  of  the  dispersion 
means  upon  the  membrane  must  be  considered.  A  possible  chem- 
ical effect  of  the  substances  subjected  to  dialysis  must  also  be  kept 
in  mind,  though  such  is  rarely  met  with  among  the  colloids. 


FIG.  47. — Thomas  Graham's  disc 
dialyzer. 


FIG.  48. — Thomas  Graham's  bell  dialyzer. 


3.  Experimental  Facts  Regarding  Dialysis  of  Colloids. — Since 
the  days  of  Graham,  almost  every  student  of  the  general  properties 
of  colloid  systems  has  made  use  of  dialysis.  It  is,  therefore,  not 
possible  to  review  all  the  work  that  has  been  done  in  this  field. 
Generally  speaking,  dialysis  teaches  the  same  facts  as  diffusion. 


FIG.  49. — A  second  method  of  using  Graham's  bell  dialyzer. 

Thus,  S.  E.  Linder  and  H.  Picton  (Lc.)  were  able  to  distinguish 
between  dialyzing  and  non-dialyzing  metallic  sulphides.  Of  the 
many  groups  of  compounds  studied,  only  one  will  be  discussed  here, 
that  of  the  technically  and  theoretically  important  water  soluble 
dyes.  F.  Krafft  and  G.  Premier,1  0.  Teague  and  B.  H.  Buxton,2 

1  F.  Krafft  and  G.  Preuner,  Ber.  d.  Dtsch.  chem.  Ges.,  32,  1620  (1899). 

2  O.  Teague  and  B.  H.  Buxton,  Z.  f.  physik.  Chem.,  60,  469  (1907). 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


225 


H.  Freundlich  and  W.  Neumann,1  R.  Hober,2  L.  Vignon,3  W. 
Biltz  and  F.  Pfenning,4  have  all  studied  these.  In  Table  43  are 
reproduced  some  of  the  findings  tabulated  by  Biltz.  In  connec- 
tion with  this  table  it  should  be  noted  that  Krafft  and  Preuner 
used  parchment  tubes;  Teague,  Buxton,  Hober  and  Vignon,  parch- 
ment-paper capsules,  manufactured  by  Schleicher  and  Schiill; 
Biltz  and  Pfenning,  collodion  membranes.  The  solutions  used  were 
usually  o.i  per  cent.;  Teague  and  Buxton  used  0.02  per  cent; 
Biltz  and  Pfenning  0.5  per  cent.  The  abbreviations  in  parentheses 
after  the  names  of  the  dyes  mark  their  origin. 

TABLE  43. — DIALYSIS  OF  DYES 
Typical  Molecular  Dispersoids 


Name 

Atomic 
number 

Molecular 
weight 

Dialyzes 

Observer 

Picric  acid 

IQ 

22O   O 

Quickly 

Vignon 

Toluidin  blue  (Hoechst)  
Naphthol  yellow  9   (Bayer, 
Hoechst)  
Chrysoidin  
Methylene  blue  

Eosin  

19 

27 
30 
37 

?7 

I43-S 

355-0 
214.0 
317.5 

602.0 

Quickly.  . 

Quickly.  . 
Quickly.. 
Quickly.  . 

Ouicklv. 

Biltz. 

Hober,  Vignon,  Biltz. 
Teague  and  Buxton. 
Krafft  and  Preuner, 
Teague  and  Buxton, 
Biltz. 
Teague  and  Buxton 

Erythrosin  . 

37 

880  o 

Quickly 

Vignon.  Biltz. 
Hober  Biltz 

Bengal  rose  

37 

1050.0 

Ouicklv. 

Hober 

Quinolin  yellow  (Akt.)  
True  acid  fuchsin  B  (Bayer)  .  . 
Auramin  o  (Akt.)  
Safranin. 

40 
4i 
43 
44 

477-0 
467.0 

303.5 
2t;o    cj 

Quickly.. 
Quickly.. 
Quickly.  . 
Quickly 

Biltz. 
Biltz. 
Biltz. 
Teague  and  Buxton 

Wool  violet  S  (Bad.)  
Brilliant  crocein  36  
Acid  fuchsin  S  (Akt.)  
Methyl  violet  

46 
51 
52 
56 

445-0 
556  o 
572.0 
•JQ7  .  e 

Quickly.. 
Quickly.. 
Quickly.  . 
Ouicklv 

Vignon,  Biltz. 
Hober. 
Hober. 
Vignon. 
Biltz 

Patent  blue  V  (Hoechst)  .... 
Guinea  green  B.. 

to 
66 
84 
86 

to 

469.5 
804.0 
73O   O 

Quickly.. 
Quickly 

Hober,  Biltz. 
Hober 

Erioglaucin 

ne 

782  o 

Quickly 

Hober 

1  H.  Freundlich  and  W.  Neumann,  Koll.-Zeitschr.,  3,  80  (1908). 

2R.  Hober,  Koll.-Zeitschr.,  3,  76  (1908);  Bioch.  Zeitschr.,  20,  80  (1909). 

3L.  Vignon,  Compt.  rend.,  150,  619  (1910). 

4  W.  Biltz  (with  F.  Pfenning),  van  Bemmelen-Gedenkboek,  108,  1910. 

IS 


226  SPECIAL    COLLOID-CHEMISTRY 

Transition  Systems  between  Molecular  Dispersoids  and  Colloid  Solutions 


Name 

Atomic 
number 

Molecular 
weight 

Dialyzes 

Observer 

Neutral  red  

37 

288.5 

Slowly 

Teague  and  Buxton. 

True  red  A  (Akt.  Bayer)  
Ponceau  2  R  (Akt.)  

Ponceau  B  O  extra  (Akt.)  .... 
Victoria  black  B  (Bayer)  
Nile  blue  .  

41 
45 

5i 

58 
58 

400.0 
480.0 

SS6.0 
622.0 
443  .4 

Slowly 
Rather 
quickly 
Rather 
quickly 
Only  in 
traces 
Slowly 

Hober,  Biltz. 
Hober,  Biltz. 

Biltz. 
Biltz. 
Teague  and  Buxton. 

Crystal  violet 

rn 

4O7    "? 

Rather 

Freundlich  and  Neu- 

Aniline blue  

74    ' 

565.5 

quickly 
Very 

mann,  Vignon. 
Teague  and  Buxton, 

Benzo  blue  3  B  (Bayer)  
Acid  violet  6  B  (Akt)  

86 

QI 

960.0 
733.O 

slowly 
Slowly 
Some- 

Hober. 
Hober. 
Hober,  Biltz. 

what 

Typical  Colloid  Solutions 


Name 

Atomic 
number 

Molecular 
weight 

Dialyzes 

Observer 

Cloth  red  6  A  (Akt.) 

C7 

482.0 

Not  at  all 

Biltz. 

Congo  brown  9  (Akt  ) 

68 

682.0 

Not  at  all 

Hober,  Biltz,  Teague, 

Congo  red  (Akt  ) 

7O 

606.0 

Not  at  all 

and  Buxton. 
Vignon,  Biltz. 

Azo  blue  (Akt.)   

74 

726.0 

Not  at  all 

Krafft  and  Preuner, 

B  enzopurpurin 

76 

724   O 

Not  at  all 

Teague  and  Buxton, 
Hober,  Biltz. 
Krafft  and  Preuner, 

Congo  blue  B  X  26  (Akt.)... 
Night-blue 

80 
84 

860.0 
575.5 

Not  at  all 
Not  at  all 

HQber,  Biltz. 
Biltz. 
Teague  and  Buxton, 

Heliotrope  B  B  (Bayer)  

88 

SlO.O 

Not  at  all 

Freundlich   and 
Neumann,  Biltz. 
Hober. 

Chicago  blue  6  B  R  W  (Akt.)  . 

88 

9Q2.0 

Not  at  all 

Biltz. 

The  table  shows  that,  in  general,  dialyzability  decreases  with 
rising  atomic  number  and  increasing  molecular  weight.  That  the 
rule  is  only  approximately  true  can  be  seen  by  comparing  the 
tables  horizontally.  (The  vertical  rows  are  arranged  accord- 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  227 

ing  to  increasing  atomic  numbers.)  In  each  of  the  three  classes, 
some  of  the  dyes  have  a  low,  while  some  have  a  high,  atomic  num- 
ber. Even  substances  with  high  molecular  weights,  as  Bengal 
Rose  may  be  found  in  the  rapidly  dialyzing  class.  The  degree  of 
dispersion  of  the  dye  is  therefore  dependent  not  alone  on  the  atomic 
number  or  the  molecular  weight,  but  on  other  factors  as  well.  It 
seems  natural  to  have  tried  to  explain  the  lack  of  parallelism  through 
the  chemical  constitution  of  the  dyes,  as  W.  Biltz  and  others  have 
done  with  a  fair  degree  of  success.  A  review  of  Biltz's  results  is 
beyond  the  limits  of  this  book,  but  it  should  be  noted  that  even  so, 
a  quantitative  relation  between  chemical  constitution  and  degree 
of  dispersion  does  not  appear  even  when  only  simple  compounds 
in  homologous  series  are  considered.  The  absence  of  parallelism 
between  size  of  particles  and  molecular  weight  demonstrates  also 
the  danger  of  trying  to  determine  molecular  weight  from  diffusion 
constants  as  discussed  on  p.  218. 

When  the  dialysis  of  non-aqueous  colloids  is  discussed  it  must 
first  be  remembered  that  many  dyes  " dissolve"  to  form  colloid 
solutions  in  water,  but  molecularly  dispersed  ones  in  other  sol- 
vents, such  as  alcohol  (F.  Krafft,  I.e.,  and  others).  Corresponding 
herewith,  the  alcoholic  solutions  dialyze  better  than  the  aqueous 
ones.  Especially  interesting  results  have  been  obtained  with 
iodine  dissolved  in  different  organic  solvents.  J.  Amann1  has 
shown  that  iodine  dissolves  in  benzene  as  a  molecular  dispersoid, 
in  petroleum  as  a  colloid.  Corresponding  to  this  fact,  it  dia- 
lyzes  through  a  parchment  capsule  out  of  its  solution  in  benzene 
but  not  out  of  that  in  petroleum.2 

4.  Special  Observations  Regarding  the  Dialysis  of  Colloids.— 
Colloids  frequently  pass  through  a  dialyzing  membrane /0r  a  short 
time  immediately  following  their  preparation.  This  is  especially  true 
of  freshly  prepared  silicic  acid  as  observed  by  Th.  Graham  and 
more  recently  confirmed  by  F.  Mylius  and  E.  Groschuff.3  The 
explanation  of  this  interesting  fact  is  to  be  found  in  the  instability 
of  the  degree  of  dispersion  in  colloid  systems.  When  a  colloid 
solution  is  prepared  by  condensation  of  a  molecularly  dispersed 
system,  the  desired  product  is  not  obtained  at  once,  but  only  after 
hours  or  days.  Sometimes,  moreover,  the  condensation  occurs 

1  J.  Amann,  Koll.-Zeitschr.,  7,  235  (1910);  7,  67  (1910). 

2  According  to  the  unpublished  results  of  Prof.  S.  Suzuki  and  the  author. 

3  F.  Mylius  and  E.  Groschuff,  Ber.  d.  Dtsch.  chem.  Ges.,  39,  119  (1906). 


228  SPECIAL    COLLOID- CHEMISTRY 

unequally,  in  other  words,  a  few  colloid  particles  are  first  pro- 
duced but  their  number  gradually  increases  with  time,  at  the 
expense  of  the  molecularly  dispersed.  It  is  to  such  changes  that 
the  behavior  of  silicic  acid,  of  many  albumin  solutions,  of  humic 
acid,  etc.,  must  be  referred. 

Another  phenomenon  of  both  practical  and  theoretical  impor- 
tance is  the  chemical  decomposition  through  dialysis  of  molecularly 
dispersed  substances  with  formation  of  a  colloid  phase.  It  was  known 
to  Graham  and  belongs  to  the  earliest  methods  of  preparing  col- 
loid systems.  It  is  essential  that  the  original  material  suffer 
hydrolysis  in  water,  yielding  an  insoluble,  or  but  slightly  soluble, 
component.  This  is  true  of  the  chlorides,  nitrates,  acetates,  etc., 
of  the  metals.  Since  the  molecularly  soluble  product  of  the  hy- 
drolysis passes  through  the  dialyzing  membrane  while  the  "  insol- 
uble" component  remains  behind  in  colloid  form,  a  continual 
displacement  of  the  hydrolysis  takes  place,  favoring  the  forma- 
tion of  the  colloid.  To  obtain  the  corresponding  colloid  hydrate 
it  is  only  necessary,  therefore,  to  place  the  proper  salt  solutions 
in  the  dialyzer. 

From  the  abundant  literature  describing  these  phenomena  we 
may  cite  the  following  example  of  the  chemical  changes  exhibited 
by  iron  hydroxide-iron  chloride  solutions,  during  dialysis,  as 
observed  by  S.  E.  Linder  and  H.  Picton.1  Table  44  shows  the 
changes  in  composition  of  the  outer  liquid  during  the  process. 

TABLE  44. — CHANGB^IN  COMPOSITION  OF  OUTER  LIQUID  DURING  DIALYSIS 

OF  IRON  HYDROXIDE-IRON  CHLORIDE  SOLUTIONS 

(According  to  S.  E.  Linder  and  H.  Picton) 

Time  of  dialysis  in  hours  Relation  of  Fe  to  HC1  in  outer  liquid 


o  56  :  109.5 

24  56  :  137.0 

48  56  :  609 . o 

120  56  :  1086  .o 

168  |    Not  demonstrable: evident 

! 

Toward  the  end  of  the  experiment,  as  can  be  seen,  only  HC1  passed 
through  the  dialyzing  membrane. 

The  changes  in  composition  of  the  inner  liquid  during  the  di- 
alysis is  shown  in  Table  45. 

1  S.  E.  Linder  and  H.  Picton,  Trans.  Chem.  Soc.  Lond.,  1909  (1905). 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


229 


TABLE  45. — CHANGES  IN  COMPOSITION  OF  INNER  LIQUID  DURING  DIALYSIS 

OF  IRON  HYDROXIDE-IRON  CHLORIDE  SOLUTIONS 

(According  to  S.  E.  Linder  and  H.  Picton) 


Time  of  dialysis  in 

Composition  in  grams  per  100  cc. 

Calculated  molecular 

days 

weight 

Fe 

Cl                       Formula 

5 

•  2303 

0.1410 

13  Fe(OH)3,  FeCl3 

1767 

n 

.2300 

O.IIIO 

2oFe(OH)3,FeCl3 

2302 

IO 

.7200 

0.1250 

25Fe(OH)3,FeCl3 

2837 

17 

.5000 

0.0773 

36  Fe(OH)3,  FeCl3 

4014 

30 

.2400 

0.0550 

42Fe(OH)3,FeCl3 

4656 

37 

.1800 

o  .  0490 

45Fe(OH)3,FeCl3 

4977 

44 

.1400 

o  .  0460 

46Fe(OH)3,FeC]3 

5084 

61 

.0400 

o  .  0430 

45Fe(OH)3,FeCl3 

4977 

210 

0.6550 

0.0150 

82Fe(OH)3,FeCl3 

8936 

Gels  separated  out 

per 

!    O.OI2C1 

i62Fe(OH)3,FeCl3               17496 

after  120  days. 

gram  Fe 

The  table  shows  plainly  the  relative  increase  in  iron  hydroxide 
content  at  the  cost  of  the  hydrochloric  acid.  The  formulas  of  the 
iron  compounds  produced  and  their  respective  molecular  weights, 
as  calculated  by  Linder  and  Picton,  are  also  given.  The  impossi- 
bility of  isolating  the  compounds,  coupled  with  the  fact  that  they 
show  a  progressive  change  makes  the  chemical  significance  of  the 
numbers  assigned  as  molecular  weights  rather  fanciful.  On  the 
other  hand,  they  demonstrate  very  well  the  progressive  transmu- 
tation to  the  colloid. 

The  progressive  decomposition  during  dialysis,  with  formation 
of  colloid  in  such  solutions  can  be  shown  in  a  striking  manner, 
according  to  N.  Sahlbom,1  by  "  capillarizing "  them,  that  is,  by 
dipping  strips  of  filter  paper  into  them.  If  this  is  done  every  24 
hours  to  ferric  chloride  or  ferric  nitrate  undergoing  dialysis,  pictures 
are  obtained  like  those  shown  in  Figs.  50  and  51.  At  the  beginning 
of  dialysis  the  molecularly  dispersed  solution  ascends  the  paper 
without  decomposition  and  concentrates  high  up,  as  shown  by  the 
dark  bands  at  the  tops  of  the  colored  columns.  After  i  or  2  days, 
the  upper  concentrated  salt  zone  begins  to  disappear  while  a  sec- 
ond less-colored  one  appears  below.  The  latter  consists  of  col- 
loid iron  hydroxide  which  when  first  formed  is  highly  dispersed. 

1  N.  Sahlbom,  Kolloidchem.  Beihefte,  2,  79  (1910). 


230 


SPECIAL    COLLOID-CHEMISTRY 


With  progressing  dialysis,  the  molecularly  dispersed  salt  dis- 
appears entirely  at  the  expense  of  the  iron  hydroxide,  which  gradu- 
ally acquires  the  properties  of  a  typical,  positively  charged  col- 


FIG.  50. — Dialysis  of  a  ferric  chloride  solution.     (According  to  N.  Sahlbom.) 


FIG.  51. — Dialysis  of  a  ferric  nitrate  solution.     (According  to  N.  Sahlbom.} 

loid  and  therefore  ascends  filter  paper  little,  if  at  all,  as  described 
on  p.  16. 

Finally,  a  third  phenomenon  often  observed  during  dialysis 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  231 

deserves  mention.  When  the  separation  of  the  molecularly  dis- 
persed or  electrolytic  components  of  a  system  from  the  colloid  is 
far  advanced,  a  radical  change  in  the  state  of  the  system  often 
occurs.  It  may  coagulate.  This  fact,  which  was  already  observed 
by  Graham,  shows  that  the  presence  of  a  certain  amount  of  elec- 
trolyte is  necessary  to  insure  colloid  stability.  An  example  of  this 
behavior  is  offered  in  Table  45,  when  the  ferric  hydroxide  has  been 
dialyzed  120  days. 

§30.  Osmosis  of  Colloid  Systems 

i.  General  Remarks  and  Literature. — During  dialysis,  an 
increase  in  the  volume  of  the  dialyzing  liquid,  in  the  interior  of  the 
cell  is  often  observed.  This  is  the  phenomenon  of  osmosis,  known 
for  a  century  and  a  half.1  Osmotic  phenomena  take  place  when- 
ever a  dispersoid  is  brought  in  contact  with  a  less-concentrated 
one  or  its  pure  dispersion  means,  under  conditions  which  do  not 
allow  of  the  "free"  diffusion  described  in  §28.  This  may 
be  accomplished  by  placing  between  them  a  so-called  semi- 
permeable  or,  better  expressed,  a  selectively  permeable  membrane, 
in  other  words,  a  device  which  gives  passage  to  the  dispersion  means, 
but  not  to  the  dispersed  phase.  These  devices  are  plainly  nothing 
more  than  such  as  were  used,  for  example,  in  the  dialysis  of  colloid 
systems,  as  described  in  the  previous  paragraphs.  In  fact,  osmotic 
phenomena  may  always  be  expected  to  appear  during  dialysis. 
Consideration  of  these  osmotic  phenomena  discloses  their  close 
connection  with  the  processes  of  diffusion  and  dialysis.  Like 
the  latter,  osmosis  represents  an  impeded  diffusion.  Osmosis,  like 
free  diffusion,  tends  toward  the  establishment  of  a  uniform  spatial 
distribution  of  dispersed  phase  and  dispersion  means.  Since,  in  the 
presence  of  a  dialyzing  membrane,  the  dispersed  phase  cannot 
wander  into  the  pure  (or  less  concentrated)  outer  dispersion  means, 
the  reverse  occurs  and  the  pure  dispersion  means  wanders  into 
the  dispersed  phase.  The  result  of  this  which  represents  the  re- 
ciprocal of  free  diffusion,  is  an  equalization,  as  far  as  possible,  of 
the  concentration  of  the  dissolved  substances  in  the  different  parts 
of  the  system. 

The  intensity  of  the  tendency  to  bring  about  a  uniform  dis- 

1  For  a  history  of  the  development  of  our  knowledge  of  osmosis  see  Wilh.  Ostwald, 
Lehrb.  d.  allg.  Chem.,  2  Aufl.,  652,  Leipzig,  1903.  . 


232  SPECIAL   COLLOID- CHEMISTRY 

tribution  of  dispersed  phase  and  dispersion  means  may  be  measured 
by  opposing  this  osmotic  leveling  process  by  the  hydrostatic  pres- 
sure of  a  water  column.  The  pressure  thus  made  evident  is  called 
the  osmotic  pressure  of  the  dispersoid.1  To  make  osmosis  possible 
it  is  immaterial  whether  the  selective  permeability  of  the  mem- 
brane is  brought  about  by  its  sieve-like  action,  which  holds  back 
mechanically  the  dispersed  phase,  or  by  its  selective  properties  as  a 
solvent  in  the  sense  that  only  the  dispersion  means  is  soluble  in  it.2 

Osmotic  pressure  and  osmotic  phenomena  like  Brownian  move- 
ment and  diffusion  velocity  are  markedly  dependent  on  the  spe- 
cific surface  of  the  dispersed  phase.  Colloid  solutions,  therefore, 
show  but  slight  osmotic  pressures,  provided  they  are  not  contami- 
nated with  molecular  or  ionic  dispersoids.  Most  colloids  can  only 
with  difficulty  be  rid  of  these  impurities  which  enter  these  systems 
in  the  process  of  their  preparation  or  are  necessary  for  their 
stability.  Such  traces  of  impurities  introduce  great  errors 
into  pressure  measurements  which  at  the  best  yield  but  small 
values.3  It  cannot,  however,  be  denied  that  many  typical  col- 
loids, especially  when  of  high  dispersion,  possess  some  osmotic 
pressure  of  their  own.  This  follows  as  a  necessary  conclusion  from 
the  existence  in  them  of  Brownian  movement  and  diffusibility. 

Measurements  of  the  osmotic  pressure  of  colloids  have  been 
made  and  discussed  at  special  length  by  W.  Pfeffer,4  H.  Picton  and 
S.  E.  Linder,5  C.  E.  Linebarger,6  E.  H.  Starling,7  C.  J.  Martin,8 
A.  Lottermoser,9  B.  Moore,  W.  H.  Parker,  H.  E.  Roaf,  L.  Adam- 

1  In  many  textbooks,  following  the  lead  of  W.  Nernst,  we  find  it  stated  that 
osmotic  pressure  is  the  "cause"  or  "force"  producing  diffusion.     This  way  of 
putting  it  is  incorrect  as  the  above  remarks  on  the  relation  of  diffusion  to  osmosis  show 
and  as  J.  J.  van  Laar  (Vortrage  uber  d.  thermodynam.  Potential  usw.  Braunschweig, 
1906)  has  long  emphasized.     The   concept  of  osmotic  pressure  stands  and  falls 
with  the  presence  and  absence  of  a  selectively  permeable  membrane.     It  contradicts 
every  correct  view  of  osmotic  pressure   to  assume  its  existence  in  the  absence 
of  such  a  membrane,  as  in  the  processes  of  free  diffusion.     It  is,  however,  correct  to 
hold  that  the  phenomena  of  diffusion,  of  osmosis  and  of  Brownian  movement  all 
spring  from  the  same  source  of  energy  as  clearly  evidenced  by  the  close  relations  and 
analogies  between  them. 

2  For  details  regarding  such  and  other  properties  of  membranes  see  the  compre- 
hensive monograph  of  H.  Zangger,  Ergebnisse  der  Physiologic,  7,  99  (1908). 

8  With  reference  to  the  view  that  the  admixed  electrolytes  may  constitute  integ- 
ral parts  of  the  colloids  see  p.  143. 

4  W.  Pfeffer,  Osmotische  Untersuchungen,  Leipzig,  1877. 

5  H.  Picton  and  S.  E.  Linder,  Journ.  Chem.  Soc.,  63,  148  (1892). 

6  C.  E.  Linebarger,  Silliman's  Am.  Journ.  Sci.,  (3),  43,  218,  416  (1892). 

7  E.  H.  Starling,  Journ.  PhysioL,  19,  312  (1805-6);  24,  317  (1899). 
8C.  J.  Martin,  Journ.   PhysioL,   20,  364   (1896). 

9  A.  Lottermoser,  Anorg.  Kolloide,  Stuttgart,  1901;  Z.  f.  physik.  Chem.,  60, 451 
(1907). 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  233 

son,  D.  Bigland,1  E.  W.  Reid,2  J.  Duclaux,3  G.  Malfitano,4  R.  S. 
Lillie,5  G.  Hiifner  and  Gansser,6  W.  M.  Bayliss,7  W.  Biltz  and  A. 
von  Vegesack8  and  others.  Only  the  more  important  of  their 
findings  can  be  touched  upon  here. 

2.  Methods  of  Measuring  the  Osmotic  Pressure  of  Colloids.— 
From  what  has  been  said  it  is  clear  that  any  dialyzing  apparatus 
may  be  used  to  measure  osmotic  pressure.  As  dialyzing  mem- 
branes, the  earlier  investigators  generally  used  parchment  paper. 
More  recently  collodion  capsules  have  been  employed.  C.  J. 
Martin  (I.e.)  used  clay  cups  impregnated  with  silicic  acid  gels; 
E.  H.  Starling  (I.e.),  the  same  impregnated  with  gelatine.  For 
details  the  recent  works  of  W.  Biltz  and  A.  von  Vegesack  should 
be  consulted.  Fig.  52,  which  represents  a  cell  used  for  osmotic 
pressure  measurements,  is  taken  from  their  publications.  Below 
is  shown  the  collodion  capsule.  Of  the  two  vertical  tubes,  one  is 
used  to  fill  the  "osmometer,"  the  other  to  record  the  pressure. 

The  greatest  source  of  error  in  the  determination  of  the  osmotic 
pressure  of  colloids  lies  in  the  disturbing  effects  of  the  presence  of 
molecularly  dispersed  phases,  especially  electrolytes.  Several 
schemes  have  been  proposed  to  obviate  the  difficulty.  Different 
investigators,  especially  B.  Moore  (with  his  collaborators)  and  J. 
Duclaux,  have  maintained  that  the  accompanying  electrolytes 
constitute  integral  parts  of  the  colloid  and  are  bound  to  it  either 
chemically  (see  Duclaux)  or  at  least  through  "adsorption."  In 
other  words,  they  hold  the  electrolytes  to  be  essential  to  the 
maintenance  of  the  colloid  state.  When  they  are  removed  the 
colloid  is  "denatured"  and,  as  has  been  observed,  "polymerized" 
into  coarsely  dispersed  particles,  even  to  the  point  of  coagula- 
tion. That  all  this  may  occur,  as  in  the  case  of  the  albumins,  must 
be  admitted,  but  it  cannot  be  stated  as  a  universal  truth.  As 

1  B.  Moore  and  W.  H.  Parker,  A  mer.  Journ.  Physiol.,  7, 261  (1902);  B.  Moore  and 
H.  E.  Roaf,  Bioch.  Journ.,  2,34  (igc6);  3,  55  (1907) ;  B.Moore  and  D.  Bigland,  ibid., 
5>  32  (I9°9)>  H.  E.  Roaf  and  L.  Adamson,  Bioch.  Journ.,  3,  422  (1908);  Journ. 
Physiol.,  39  (1909);  Quart.  Journ.  Physiol.,  3, 75,  171  (1910);  in  part  available  only  in 
abstract. 

2  E.  W.  Reid,  Journ.  Physiol.,  31,  439  (1904);  33,  12  (1905). 

3  J.  Duclaux,  Compt.  rend.,  140,  1468,  1544  (1905);  Journ.  Chim.  physique,  5,  40 
(1907);  i,  407  (1909);  see  also  the  review  in  Koll.-Zeitschr.,  3,  126  (1908). 

4  G.  Malfitano,  Compt.  rend.,  142,  1418  (1906). 

5  R.  S.  Lillie,  Amer.  Journ.  Physiol.,  20,  127  (1907). 

6  G.  Hiifner  and  Gansser,  Engelmann's  Arch.  f.  Physiol.  209  (1907). 

7  W.  M.  Bayliss,  Proc.  Roy.  Soc  ,  81,  269  (1909);  Koll.-Zeitschr.,  6,  23  (191). 

8  W.  Biltz  and  A.  von  Vegesack,  Z.  f.  physik.  Chem.,  68,  357  (1909);  73,  481 
(1910). 


234 


SPECIAL    COLLOID-CHEMISTRY 


R.  S.  Lillie  (I.e.)  has  emphasized,  the  presence  of  electrolytes  is 
not  essential  to  the  existence  of  all  metallic  hydrosols,  and  no 
reason  can  be  assigned  at  present  why  one  phase  cannot  be  divided 
into  another  to  the  point  of  colloid  dispersion  in  the  entire  absence 
of  any  electrolyte.  These  remarks  are  not  intended  to  deny  the 
existence  of  colloid-electrolyte  complexes.  They  are  only  made 
to  emphasize  that  such  discussion  does  not  an- 
swer the  question  of  what  is  the  value  of  the  os- 
motic pressure  of  pure  colloids  themselves  and 
how  it  may  be  measured,  for  theoretically  the  col- 
loids must  have  some  because  they  show  Brownian 
movement  and  diffuse. 

The  following  measures  have  been  proposed 
to  attain  this  end.  At  first  sight  it  would  seem 
most  satisfactory  to  use  membranes  which  per- 
mit a  sharp  dialytic  separation  of  colloids  and 
molecular  dispersoids.  In  the  course  of  the  dialy- 
sis the  molecular  dispersoids  would  then  pass 
through  the  membrane  while  the  colloids  would 
remain  behind.  The  end  pressure  would  then 
be  that  of  the  pure  colloid.  It  is  well  to  empha- 
size, at  once,  that  these  final  osmotic  pressures 
have  almost  invariably  been  found  to  be  very  low. 
A  second  method  consists  in  taking  a  limited 
volume  of  outer  liquid  and  waiting  until  an  equi- 
librium has  been  established  between  the  concen- 
tration of  the  electrolytes  in  this  and  the  con- 
centration of  those  contained  in  the  inner  liquid. 
In  connection  with  this  method  it  must  be  borne 
in  mind  that  the  equilibrium  need  not  in  any 
sense  be  synonymous  with  equality  of  concentration  in  the  two 
liquids.  A  whole  series  of  facts,  one  of  which  is  the  difficulty  of 
"washing  out"  the  last  traces  of  electrolytes  from  precipitates, 
compels  the  conclusion  that  colloids  tend  to  concentrate  electrolytes 
upon  themselves1  and  thereby  to  increase  the  possibility  of  de- 
veloping and  exhibiting  a  greater  osmotic  pressure  than  is  really 
due  to  the  colloids  themselves.  Since  such  increases  in  concen- 
tration depend,  as  a  rule,  only  on  the  concentration  and  not  on 
the  absolute  amounts  of  the  electrolyte  present,  they  undergo 


FIG.  52— Os- 
mpmeter  of  W. 
Billz  and  A.  von 
Vegesack. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  235 

progressive  variation  as  osmosis  takes  place  because  of  the 
movement  of  the  liquid,  and  thus  further  complicate  the 
problem. 

The  following  procedure  has  also  been  used.  After  the  electro- 
lyte content  of  a  colloid  has  been  determined  by  analytical  means, 
an  amount  is  added  to  the  outer  liquid  to  bring  its  concentration 
up  to  that  assumed  to  exist  within  the  colloid.  The  overplus 
of  osmotic  pressure  exhibited  by  the  colloid  mixture  is  then  re- 
garded as  the  osmotic  pressure  of  the  colloid  itself.  To  get  a 
proper  outer  solution  the  dialysate  or  outer  liquid,  rich  in  electro- 
lytes, is  used  against  the  original  mixture,  or  a  proper  outer 
fluid  is  obtained  by  a  nitration  (see  the  following  paragraphs) 
which  separates  the  electrolyte  solution  from  the  colloid  (J. 
Duclaux,  I.e.).  Finally,  the  maximal  pressures  observed  in  the 
osmosis  of  a  colloid  solution  containing  electrolytes  has  been 
taken  as  a  convenient  method  of  arriving  at  the  osmotic  pressure 
of  the  colloid  itself.  As  W.  Biltz  and  A.  von  Vegesack  (I.e.) 
have  pointed  out,  this  is  the  resultant  of  two  processes :  of  the 
osmosis  directed  toward  the  inner  liquid  (endosmosis)  and  of 
that  directed  toward  the  outer  (exosmosis),  which  latter  parallels 
dialysis. 

These  remarks  make  it  clear  that  the  methods  for  the  quanti- 
tative determination  of  the  osmotic  pressure  of  colloid  systems  are 
not  as  yet  worked  out  entirely.  If  we  do  not  wish  to  determine 
the  osmotic  pressures  of  highly  purified  colloids  or  their  final 
values  to  the  point  of  utilizing  a  microscope  to  make  readings 
and  a  micro-osmometer,  then  employment  of  a  constant  volume  of 
outer  liquid,  with  attainment  of  an  equilibrium  between  the  elec- 
trolytes present  in  both  liquids,  seems  most  expedient.  It  would, 
of  course,  be  well  to  determine  also  the  distribution  of  the 
electrolytes  between  colloid  and  pure  dispersion  means,  in 
order  to  work  out  from  the  obtained  values  a  proper  equilibrium 
curve1  from  which  might  then  be  exterpolated  the  osmotic  pres- 
sure of  the  colloid  when  the  concentration  of  the  electrolyte 
equals  zero. 

3.  Instability  of  Osmotic  Pressure  of  Colloids. — One  of  the 
first  things  to  be  noticed  when  the  osmotic  pressures  of  colloids 
are  measured,  even  though  every  effort  is  made  to  keep  constant 
1  This  would  undoubtedly  take  the  form  of  the  adsorption  isotherms. 


236 


SPECIAL   COLLOID-CHEMISTRY 


all  external  conditions,  is  their  inconstancy.  Not  only  do  prepa- 
rations of  one  and  the  same  substance,  prepared  by  different 
methods,  show  different  osmotic  pressures,  but  shaking,  stirring, 
standing,  etc.,  all  cause  considerable  change  in  them.  The  follow- 
ing examples  illustrate  this  behavior. 

TABLE  46. — INFLUENCE  OF  PREVIOUS  TREATMENT  ON  OSMOTIC  PRESSURE 

OF  ALBUMIN 
(According  to  E.  W.  Reid) 


Previous  treatment 

Ash. 
per  cent. 

Osmotic  pressure  of 
a  I  per  cent,  solution 
in  mm.,  Hg. 

Ovalbumin,  4wice  crystallized  and  once  washed.  . 
Ovalbumin,  washed  repeatedly 

0.120 

o  267 

3.38 
O   OO 

Ovalbumin,  precipitated  and  once  washed  

O.  312 

4.82 

The  same  

O    22O 

1^.71 

Precipitated  bovine  serum-albumin,  repeatedly 
washed. 
The  same,  once  washed  

0.633 

o  461 

o.oo 

4.2O 

These  experiments  of  E.  W.  Reid  (I.e.)  show  that  the  osmotic 
pressure  of  one  and  the  same  substance  (egg-albumin)  varies  at 
the  same  concentration  between  the  values  zero  and  15.71  mm. 
of  mercury.  They  also  betray  the  important  fact  that  the  ash 
content  of  a  colloid  is  not  fundamentally  responsible  for  the  value  of 
its  osmotic  pressure.  The  osmotic  pressure  of  a  preparation  hav- 
ing the  greatest  ash  content  is  zero,  for  example. 

The  following  example,  taken  from  R.  S.  Lillie  (I.e.)  is  intro- 
duced to  illustrate  the  influence  of  shaking. 

TABLE  47. — INFLUENCE  OF  SHAKING  ON  OSMOTIC  PRESSURE  OF  GELATINE 

AND  OF  EGG-ALBUMIN 
(According  to  R.  S.  Lillie) 


1.  25  per  cent,  gelatine 

Pressure  in 
mm.,  Hg. 

1.6  per  cent,  egg-albumin              -inriSU?Ii?n 

Pure  gelatine.  .                       ' 

42 

Pure  albumin                             32.1 

Pure  gelatine  shaken  i 

5-3 

Pure  albumin  shaken.  ...         31.3 

Gelatine  +  *?  NaCl  
48 

2.6 

Albumin  -f-    0  NaCl  9.0 
45 

Gelatine  +    ~  NaCl,  shaken 

2.9 

Albumin  +  ^  NaCl,                   8.8 

48 

48        shaken 

Gelatine  +  ™  Na2SO4  

2.4 

Albumin  +  ™  Nal  1          8.9 
48 

Gelatine  +  m  Na2SO4 

2.6 

Albumin  +  ^  Nal,                    6.6 

shaken 


48 


.    . 
shaken 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


237 


It  is  a  remarkable  fact  that  while  the  osmotic  pressure  of 
gelatine  is  increased  by  shaking,  that  of  egg-albumin  is  decreased. 

Table  48  illustrates  the  influence  of  stirring  on  the  osmotic 
pressure  of  colloid  solutions. 

TABLE  48. — INFLUENCE  OF  STIRRING  ON  OSMOTIC  PRESSURE  OF  BENZOPUR- 

PURIN  SOLUTIONS 
(According  to  W.  Biltz  and  A.  von  Vegesack) 


A.  Benzopurpurin  low  in  electrolytes 

B.  Benzopurpurin  high  in  electrolytes 

Height  of 

Height  of 

Hours 

fluid                     Remarks 

Hours 

fluid 

Remarks 

column 

column 

1  .0 

9.41 

Not  stirred  

5 

1.22       Stirred. 

2-5 

9.62 

Not  stirred  

15             1.25       Not  stirred. 

3-5             9-So 

Not  stirred  

18 

1-34 

Stirred. 

4-5             9-68 

Stirred  5  min  

378 

1.30 

Stirred  i  hr.  daily. 

5.0             9.86 

Stirred  5  min  

5-5           10-07 

Stirred  5  min  

426             1.24 

Stirred  7  hrs.  pre- 

6.0          10.18 

Stirred  5  min  

viously. 

7.0           10.40 

Stirred  5  min  

•  • 

8.0           10.60 

Stirred  5  min  

4So 

1.26       Stirred  7  hrs.  pre- 

9.0          10.64 

Stirred  5  min  

viously. 

10.  o           10.66 

Stirred  5  min  

20.  o 

8.16       Not  stirred  

20.5 

8.37 

Stirred  5  min  

21.0                  8.92 

Stirred  5  min  

o 

21-5 

9.08 

Stirred  5  min  

IOO 

1.14 

Stirred  during  day. 

22.0                  9.16 

Stirred  5  min  

121 

1.19 

Stirred  6  hrs. 

23.0 

9.29 

Stirred  5  min  

145 

1.09 

Stirred  6  hrs. 

24.0 

8.98 

Stirred  5  min  

I67 

1.  10 

Stirred  6  hrs. 

25.0             8.99       Stirred  5  min  

28.0            7.16     |  Not  stirred. 

28  .  5             7  .  45       Stirred  5  min  

This  table  shows  an  increase  in  osmotic  pressure  with  every 
stirring,  even  though  the  effect  is  but  transitory.  The  increase 
occurred  three  times  in  the  data  given.  It  is  also  apparent  that 
solutions  containing  small  amounts  of  electrolytes  are  more  sensi- 
tive to  this  influence  than  those  richer  in  these  which  are  scarcely 
affected.  Gelatine  behaves  similarly,  as  shown  in  Table  47. 

In  discussing  the  influence  of  time  upon  the  osmotic  pressure 
of  colloids  we  need  to  distinguish  between  its  variations  when  a 
colloid  is  simply  left  to  itself  in  an  osmometer  and  its  variations 
if  the  same  colloid  is  measured  at  different  periods.  The  first 


238  SPECIAL    COLLOID-CHEMISTRY 

relation  is  evidenced  in  the  left-hand  column  of  Table  48.  This 
benzopurpurin  showed  a  rise  to  1.21  cm.  after  310  hours;  while 
the  capillary  rise  in  a  similar  tube  amounted  to  1.12  cm.  The 
osmotic  pressure  was  therefore  0.09  cm.  In  illustration  of  the 
influence  of  age  upon  the  solutions,  these  authors  found  a  dialyzed 
solution  of  0.00103  normal  night-blue  to  yield  a  maximum  osmotic 
pressure  of  15.52  cm.  of  water  after  2  days;  after  6  days,  4.24  cm.; 
and  after  n  days,  4.08  cm. 

When  we  survey  these  facts  we  are  struck  by  the  great  in- 
constancy of  the  osmotic  pressure  of  colloids  as  compared  with 
that  of  molecularly  dispersed  solutions.  The  osmotic  pressure 
of  colloids  is  variable,  being  greatly  modified  by  mechanical  treat- 
ment, age,  etc.  Such  sensitiveness  is  unknown  in  molecular  dis- 
persoids.  It  is  true,  of  course,  that  the  experiments  of  W.  Spring1 
have  shown  that  even  ordinary  salt  solutions,  for  example,  are 
not  absolutely  stable  in  their  conductivity,  their  optical  properties, 
etc.,  but  these  variations  are  very  small  when  compared  with 
those  exhibited  by  colloids.  The  reasons  for  this  great  variability 
are  to  be  sought  in  the  changes  of  state  of  colloids,  such  as  varia- 
tions in  their  degrees  of  dispersion,  states  of  aggregation,  etc.,  for 
which  many  different  causes  may  be  responsible,  as  will  be  dis- 
cussed later.  The  osmotic  pressure  of  colloids,  more  especially 
of  emulsoids,  varies  therefore  as  does  their  viscosity. 

4.  Influence  of  Concentration  on  Osmotic  Pressure  of  Colloids. 
— The  osmotic  pressure  of  molecular  dispersoids,  as  is  well  known, 
is  governed  by  the  important  law  of  Pfeffer-van't  Hoff :  the  osmotic 
pressure  is  directly  proportional  to  the  concentration.  The  rela- 
tions in  colloid  systems  are  not  so  simple.  Examples  are  known, 
in  which  the  law  holds  approximately,  but  there  are  also  those  in 
which  the  osmotic  pressure  increases  faster  than  the  concentration, 
or  more  slowly  than  this.  Perhaps  nothing  better  demonstrates 
the  inappropriateness  of  applying  without  due  consideration,  the 
"solution  laws"  valid  for  molecularly  dispersed  systems  to  colloid 
systems,  than  this  variability  of  the  concentration  function  of  the 
osmotic  pressure  of  colloids. 

The  following  findings  of  W.  Biltz  and  A.  von  Vegesack  (I.e.) 
on  purified  congo  red  may  serve  to  illustrate  the  first  of  the  three 

1  W.  Spring,  Koll.-Zeitschr.,  7,  22  (1910),  where  references  to  earlier  papers  on  this 
subject  may  be  found. 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


239 


possibilities,  namely,  that  wherein  concentration  and  osmotic 
pressure  are  approximately  proportional.  W.  M.  Bayliss  (Lc.) 
also  noted  this  proportionality  in  concentrations  ranging  from 
0.07  to  i  per  cent,  by  weight. 

TABLE  49. — RELATION  OF  OSMOTIC  PRESSURE  TO  CONCENTRATION  IN  DIALYZED 

CONGO  RED  SOLUTIONS 
(According  to  W.  Biltz  and  A.  von  Vegesack) 


Concentration 
C 

Osmotic  pressure  in  cm. 
P 

^r  =  const. 

0.539  norm. 

4.15  cm. 

0.770 

i.  08 

8.15  cm. 

0-755 

1-44 

10.24  cm- 

0.695 

i.  80 

14.00  cm. 

0.778 

2.155 

14.62  cm. 

0.678 

2.87 

18.70  cm. 

0.652 

3.23 

21.55  cm- 

0.667 

3-59 

25  .  04  cm. 

0.698 

4-31 

25.30  cm. 

0.587 

The  constants  are  all  of  about  the  same  order  of  magnitude. 
Gum  arabic  behaves  similarly  according  to  W.  Pfeffer  (Lc.). 

TABLE  50. — OSMOTIC  PRESSURE  OF  GUM  ARABIC  IN  DIFFERENT  CONCENTRATIONS 
(According  to  W.  Pfeffer) 


Concentration 

Osmotic  pressure  in  cm.,  Hg. 
P 

P 
C 

i  per  cent. 

6-59 

6.9 

6 

25-9 

4.3 

14 

70.0 

5-0 

8 

119.0 

6.6 

The  observations  of  J.  Duclaux  (I.e.)  on  the  same  substance 
and  given  in  Table  51  should  be  compared  with  these. 

Some  illustrations  of  how  the  osmotic  pressure  may  increase 
more  rapidly  than  the  concentration  are  given  in  Table  51. 

P 

As  readily  apparent,  the  relation  —  increases  greatly  with  rising 

o 

concentration.  This  is  altogether  different  from  the  behavior 
of  molecular  dispersoids,  in  which,  so  far  as  known,  the  opposite 
occurs  as  the  concentration  rises.  That  the  experimental  methods 


240 


SPECIAL    COLLOID-CHEMISTRY 


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O       H       M       d       d 

W 

•^,  o 

3 

1 

1 

I?  v5  5 

ra 

8 

.    O     to 
g    <N    to    O     O     O 

OO    OO    VO     to  VO 

f 

O     O     c*i    t^    cs 

O     d     to    d     d 

A 

d 

M       d 

1 

M 

to    «      ^  OO      rf 

e    M.     O     O     O   °^ 

So                   M 

00     Tt-    to    to  vo 
O     O     O     co    co 

M     d     co    co  00 

0 

*j 

of 

§5     5     5    S 

MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


241 


used  by  Duclaux  are  not  responsible  for  this  behavior  is  shown 
by  the  experiments  of  W.  Biltz  and  A.  von  Vegesack  (I.e.)  included 
in  the  table  as  controls. 

A  relative  decrease  in  osmotic  pressure  with  increasing  con- 
centration, a  relation  typical  of  concentrated  molecular  disper- 
soids,  has  been  observed  by  B.  Moore  and  H.  Parker  (I.e.)  in 
sodium  oleate  solutions.  Thus  a  0.5  per  cent,  solution  showed  a 
maximum  osmotic  pressure  of  14.4  mm.  (at  55°),  while  a  3  per 
cent,  solution  showed  one  of  37.2  mm.  (at  40°).  The  two  quo- 

P 

tients,  -~,  are  288  and  124  respectively.     In  other  words,  a  six- 
\s 

fold  increase  in  concentration  caused  only  a  two  and  one-half 
fold  increase  in  osmotic  pressure.  Table  52,  containing  the 
exceedingly  careful  experimental  results  of  E.  W.  Reid  (I.e.) 
on  the  osmotic  pressure  of  repeatedly  crystallized  hemoglobin, 
illustrates  very  strikingly  what  has  been  said. 


TABLE  52. — RELATION  OF  OSMOTIC  PRESSURE  OF  HEMOGLOBIN  SOLUTIONS 
TO  THEIR  CONCENTRATION 
(According  to  E.  W.  Reid) 


Concentration 

Temperature 

Osmotic  pressure 
in  mm.,  Hg. 

Osmotic  pressure 
per  i  per  cent, 
hemoglobin 

1 

"  C 

2  .  76  per  cent. 

14-5° 

1 
12  mm. 

4-35 

2.92 

IS 

12 

4.  ii 

4.58 

15 

17 

3-7i 

4-95 

15 

19                              3.84 

5-70                                    15 

17                              3-Si 

6-05                                    15 

22                                             3.63 

6.07                                    15 

23 

3-79 

Disregarding  some  slight  irregularities,   the  decrease  in  the 
quotients  is  unmistakable.     The  differences  are  brought  out  most 

plainly  if  the  variation  of  the  quotient,  ~,  is  represented  graph- 
ically. This  has  been  done  (in  arbitrary  units)  in  Fig.  53.  The 
three  different  types  are  easily  recognized. 

It  should  be  mentioned  that  J.  Duclaux  (I.e.,  1910)  has  observed 
a  minimum  for  the  quotient  in  the  case  of  Berlin  blue,  though 
16 


242 


SPECIAL   COLLOID-CHEMISTRY 


he  has  himself  raised  some  doubts  as  to  the  reliability  of  his 
measurements. 1 

The  theoretical  significance  of  these  different  concentration 
curves  we  shall  discuss  later  (see  p.  257). 

5.  Influence  of  Temperature  on  Osmotic  Pressure  of  Colloids. 
— C.  J.  Martin  and  W.  M.  Bayliss  (I.e.)  state  that  the  osmotic  pres- 


Hemocflobin 


Concentration  — > 

FIG.  53. — Relation  between  concentration  in  colloid  systems  and  the  quotient  of  the 
osmotic  pressure  and  concentration. 

sure  of  albumin,  hemoglobin  and  congo  red  varies  rectilinearly 
with  the  temperature,  in  other  words,  directly  with  the  absolute 
temperature.  This  statement  would  make  Gay-Lussac's  law 
valid  for  these  solutions.  The  findings  of  B.  Moore  and  Roaf 
(I.e.),  J.  Duclaux  (l.c.),  W.  Biltz  and  A.  von  Vegesack  (I.e.), 
1  The  measurements  of  W.  Pfeffer  on  gum  arabic,  given  in  Table  50,  also  show  a 
minimum  value  for  the  quotient  * 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  243 

etc.,  contradict  this.  Moore  and  Roaf  found  the  osmotic  pressure 
of  gelatine  solutions  to  increase  considerably  faster  than  the 
absolute  temperature.  Technical  night-blue  solutions  show  an 
analogous  behavior,  according  to  the  figures  of  W.  Biltz  and 
A.  von  Vegesack,  contained  in  Table  53. 

TABLE  53. — INFLUENCE  OF  TEMPERATURE  ON  OSMOTIC  PRESSURE  OF  A  3.49 
NORMAL  SOLUTION  OF  TECHNICAL  NIGHT-BLUE 
(According  to  W.  Biltz  and  A.  von  Vegesack) 


Temperature, 

t°  r° 


Osmotic  pressure  in  cm. 
P 


0 

273 

6.21 

O.O22 

25 

298          10.81* 

0.036 

50 

323 

13-83 

0.043 

70 

343 

17.69 

0.050 

*  Average  of  two  experiments. 

J.  Duclaux  has  observed  the  opposite  to  be  true  of  iron  hydrox- 
idesol.  In  this,  the  osmotic  pressure  decreases  not  only  rela- 
tively, but  even  absolutely,  with  rising  temperature.  There  exists 
no  analogue  for  this  in  the  field  of  molecular  dispersions.  Duclaux 
found  the  following: 

Temperature 2°  (275)  25°  (298)  70°  (343) 

Osmotic  pressure  (cm.) 22.9  21.3  20.9 

P 

j, 0.083  0.071  0.061 

Figure  54  shows  graphically  how  differently  the  osmotic  pres- 
sure of  different  colloids  varies  with  changes  in  the  temperature. 
The  dotted  line  represents  the  ideal  case  in  which  there  exists 
simple  proportionality  between  the  two  as  is  the  case,  at  least 
approximately,  in  molecular  dispersoids. 

It  should  now  be  pointed  out  that  B.  Moore  and  Roaf  (I.e.) 
and  R.  S.  Lillie  (I.e.)  observed  interesting  thermal  after-e/ects 
or  so-called  hysteresis  phenomena  in  gelatine  solutions.  Thus 
gelatine  solution  which  has  been  heated  continues  to  show  a 
higher  osmotic  pressure  for  some  time  after  cooling  than  when 
kept  continuously  at  the  lower  temperature.  The  following  Table 
54  taken  from  R.  S.  Lillie  illustrates  this.  It  also  shows  that  the 
differences  first  noted  between  the  previously  cooled  and  the  pre- 
viously warmed  gelatine  become  less  with  time. 


244 


SPECIAL   COLLOID-CHEMISTRY 


TABLE  54. — INFLUENCE  or  THERMAL  HISTORY  ON  OSMOTIC  PRESSURE  OF 

i  PER  CENT.  GELATINE 

(According  to  R.  S.  Lillie) 

Osmotic  Pressure  at  Room  Temperature  in  Mm.  Hg. 


Age  of  the  solutions 

| 
Previously  chilled  on  ice 

Previously  warmed  to  65-70° 

i 

i  day 

5-0                                                6.4 

5  days 

5-0                                       5-3 

2  days 

4.9  (chilled                          6.0 

for  long  time  previously) 

i  day 

S-7 

6.2 

i  day 

5.6                                         6.0 

Iron  hydroxide  sol 


Niqhh 


blue 


Temperature 


25  50  70° 

FIG.  54. — Relation  between  the  temperature  of  colloids  and  the  quotient  of  osmotic 
pressure  and  absolute  temperature. 

This  behavior  also  is  unknown  among  the  molecular  dis- 
persoids. 

6.  Influence  of  Added  Substances  on  Osmotic  Pressure  of 
Colloids. — The  influence  of  added  substances  upon  the  osmotic 
pressure  of  a  given  system,  is,  according  to  the  classic  theory  of 
molecularly  dispersed  solutions,  purely  additive.  In  other  words, 
the  pressure  exerted  by  the  added  substance  is  added  to  that  of 
the  original  system.  There  exist  exceptions  to  this  rule,  of 
course,  and  usually  in  the  sense  that  the  calculated  osmotic 
pressures  are  found  to  be  greater  than  those  actually  observed. 

The  effect  of  added  substances  on  the  osmotic  pressure  of 
colloid  systems  is  more  complicated.  Under  this  heading  also, 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


245 


concentration  and  temperature  functions  are  encountered  which 
not  only  do  not  correspond  with  any  observed  among  molecular 
dispersoids,  but  which  among  themselves  show  great  differences. 

The  influence  of  added  substances  may  be  studied  by  adding 
them  in  equal  concentration  to  both  the  inner  and  the  outer 
liquid.  The  important  experiments  of  R.  S.  Lillie  (I.e.)  were 
carried  out  in  this  way. 

Acids  and  alkalies  may  either  increase  or  decrease  the  osmotic 
pressure  of  different  colloids.  Sometimes  one  and  the  same 
colloid  may.  show  both  types  of  behavior.  Often  very  small 
quantities  of  hydrogen  or  hydroxyl  ions  are  sufficient  to  cause 
noticeable  effects.  W.  M.  Bayliss  (I.e.)  found  the  osmotic  pres- 
sure of  very  pure  (and  highly  dispersed)  congo  red  to  fall  from  207 
mm.  to  120  mm.  when  the  outer  water  (conductivity  water) 
surrounding  his  osmometer  was  replaced  by  the  same  water  satu- 
rated with  carbon  dioxide.  The  stronger  acids  produce,  of 
course,  still  more  marked  effects.  The  addition  of  alkali  in- 
creases the  osmotic  pressure  until  a  maximum  is  reached,  beyond 
which  it  falls  again.  Table  55  gives  a  part  of  R.  S.  Lillie' s  find- 
ings on  gelatine.1 

TABLE  55. — INFLUENCE  OF  ACIDS  AND  ALKALIES  ON  OSMOTIC  PRESSURE  OF 
1.5  PER  CENT.  GELATINE 
(According  to  R.  S.  Lillie) 


Influence  of  HC1 

Influence  of  KOH 

Concentration 

Osmotic  pressure 
in  mm.  Hg. 

Concentration 

Osmotic  pressure 
in  mm.  Hg. 

o 

8.2 

o 

7-9 

0/3100  HC1 

6.8 

n/3ioo  KOH 

I4.I 

11/2050 

12.3 

n/62o 

.    23.7 

n/i5So 

17.9                 n/4i2 

25.1 

n/IO24 

26.5 

n/3io 

2Q.O 

0/770 

32.4 

0/620 

34-9 

n/4i2 

39-3 

As  can  be  seen,  low  concentrations  of  acid  lead  to  a  slight  but 
definite  minimum  of  osmotic  pressure.     With  higher  concentra- 

1  See  also  the  analogous  findings  of  H.  E.  Roaf  (I.e.)  on  hemoglobin. 


246 


SPECIAL    COLLOID-CHEMISTRY 


tions,  there  occurs  a  sharp  increase  in  osmotic  pressure  which  rises 
steadily  for  a  time  with  increasing  concentration.  R.  S.  Lillie 
thinks  it  probable  that  beyond  a  certain  point  a  decrease  in  os- 
motic pressure  would  again  occur.  Within  the  concentration 
range  studied,  alkalies  led  only  to  an  increase  in  osmotic  pressure. 
Figure  55  shows  graphically  this  variation  of  the  osmotic 


0.00/1 


.0020 

Concen  IraHon 


normal 


FIG.  55. — Effect  of  acid  and  alkali  upon  the  osmotic  pressure  of  a  1.5  per  cent, 
gelatine  solution.     (According  to  experiments  by  R.  S.  Lillie.) 

pressure  of  gelatine  solutions  with  the  concentration  of  the  added 
acids  and  bases. 

Contrary  to  the  findings  in  the  case  of  gelatine,  the  osmotic 
pressure  of  egg  albumin  is  always  lessened  by  the  addition  of 
hydrogen  or  hydroxyl  ions.  Table  56  shows  this. 

In  the  case  of  the  acids  a  definite  minimum  again  appears. 
The  type  of  curve,  at  least  for  albumin,  is  therefore  not  so  funda- 
mentally different  from  that  for  acid  gelatine. 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


247 


TABLE  56. — INFLUENCE  OF  ACIDS  AND  ALKALIES  ON  OSMOTIC  PRESSURE  OP 

1.5  PER  CENT.  EGG  ALBI  MIN 

(According  to  R.  S.  Lillie) 


HC1 

KOH 

Concentration 

Osmotic  pressure 
in  mm.  Hg. 

Concentration 

Osmotic  pressure 
in  mm.  Hg. 

0 

25.6 

0 

25.6 

0/3100  HC1 

20.7 

n/3ioo  KOH 

24.1 

n/I24O 

ii.  S             j     0/1240 

22.6 

n/62o 

14.1 

n/62o 

•20.2 

0/412 

20.4 

n/4i2 

18.0 

n/sio 

22.2 

n/3io 

17.9 

0.0'09        .0)8  .0^6  normal 

Concenl-naHon *• 

FIG.  56. — Effect  of  acid  and  alkali  on  the  swelling  of  gelatine  plates.    (According  to 
experiments  by  Wo.  Ostwald.) 

To  illustrate  the  varied  influence  of  salts  the  following  examples 
may  be  given.    Technical  night-blue  contains  a  considerable  ad- 


248 


SPECIAL   COLLOID-CHEMISTRY 


mixture  of  electrolytes.     If   they  are  removed  by  dialysis  its 
osmotic  pressure  increases,  as  shown  in  the  following  table. 

The  behavior  of  gelatine  and  albumin  toward  added  salts  has 


Acid 


0.01 


.02 


.03  norms/ 


'Acid 


Alkali 


0.001 


.002 
Concentration 


003  normal 


FIG.  57. — Relation'between  internal  friction  (upper  figure)  and  osmotic  pressure 
(lower  figure)  in  albumin  solutions  when  acid  or  alkali  is  added  in  different  concentra- 
tions. (From  experiments  by  Wo  Pauli  and  his  coworkers  and  R.  S.  Little.')  Only 
the  percentage  increase  in  viscosity  and  not  its  absolute  value  could  be  given  in  the 
upper  figure. 

been  extensively  studied  (B.  Moore  and  coworkers,  R.  S.  Lillie, 
etc.).  The  following  general  truths  are  taken  from  the  findings 
of  R.  S.  Lillie: 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


249 


TABLE  57. — OSMOTIC  PRESSURE  OF  PURIFIED  NIGHT-BLUE  AND  OF  NIGHT-BLUE 

CONTAINING  ELECTROLYTES 
(According  to  W.  Biltz  and  A.  von  Vegesack) 


Purified  colloid 


Colloid  containing  electrolytes 


Concentration 

Osmotic  pressure 
in  cm. 

Concentration 

Osmotic  pressure 
in  cm. 

1.30 

5-8l 

1.20 

4.72 

1.74 

12.70 

I.S8 

S.IO 

2.17 

16.64 

1.  60 

S.3I 

2.6l 

21.99 

1.96 

6.24 

3-04 

20.24 

2.36 

7.90 

3.QI 

2S-32 

2-73 

9.42 

4-34 

32.18 

3-49 

11.19 

5-21 

37-24 

5.76 

14.10 

6.08 

43-94 

6.12 

20.  8l 

W.  M.  Bayliss  (I.e.)  obtained  analogous  results  for  congo  red. 

The  addition  of  salts  always  causes  a  decrease  in  the  osmotic 
pressure  of  these  colloids.  The  degree  of  this  decrease  varies 
with  the  concentration  and  with  the  nature  of  the  anion  and 
cation.  Generally  speaking,  the  neutral  salts  of  the  alkali  metals 
cause  the  smallest  decrease.  The  salts  of  the  alkaline  earths  are 
more  effective  and  those  of  the  heavy  metals  most  effective  of 
all,  though  they  vary  considerably  among  themselves.  With 
salts  having  a  common  cation  the  order  of  the  anions,  when  that 
most  effective  is  given  first,  is  about  as  follows: 

S04>Cl>N03>Br>I>CNS1 
The  cations  similarly  arranged  follow  the  order: 

heavy  metals  >  alkaline  earths  >  alkali  metals. 

Table  58  details  some  of  the  actual  experimental  findings.  If 
the  validity  of  the  above-mentioned  conclusions  is  to  be  tested, 
the  data  of  the  original  papers  must  be  consulted  for  the  experi- 
ments differ  considerably  among  themselves. 

Figures  58  and  59  also  show  the  complicated  effects  of  the  con- 
centration of  the  added  salts  upon  the  osmotic  pressure.  The 
original  paper  (I.e.,  p.  in)  must  be  consulted  for  the  detailed 
data  upon  which  these  figures  are  based. 

1  For  similar  findings  on  hemoglobin  see  the  work  of  H.  E.  Roaf  (I.e.). 


25° 


SPECIAL   COLLOID-CHEMISTRY 


TABLE  58. — INFLUENCE  OF  SALTS  ON  OSMOTIC  PRESSURE  OF  COLLOIDS 
(According  to  R.  S.  Lillie) 


Salts  of  the  alkalies                                              Salts  of  the  alkaline  earths 

1.25  per  cent,  albumin 

i.  25  per  cent,  gelatine 

i.  2  5  per  cent,  albumin 

1.25  per  cent,  gelatine 

•5 

a 

.5 

.3 

§ 

g 

a 

S 

g 

s 

a 

S 

k 

1 

S3 

d 

« 

1 

S3 

1 

M 

o 

°g 

• 

0 

Is" 

o 

° 

° 

o 

oti 

0 

o 

O 

o 

O 

21.6 

o 

7.9 

O 

21.5 

0 

5.9 

m/24  NaCl 

S-o 

m/24  KC1 

3.3 

m/96  MgCl2 

7-3 

m/96  MgCl2 

3.2 

m/24  NaBr 

4.6 

m/24  KBr 

3-7 

111/96  CaCl2 

7-6 

m/96  CaCl2 

2.7 

m/24  Nal 

4.0 

m/24  KI 

3.7 

m/96  SrCl2 

7.2 

m/96  SrCl2 

3.1 

m/24  NaNO3 

4-8 

m/24  KNO3 

3-5 

m/96  BaCl2 

7.6 

m/96  BaCl2 

2.7 

m/24  NaCNS 

S-3 

m/24  KC1O3 

3-7 

m/24  Na2SO4 

4.0 

m/24  KBrO3 

3-6 

m/24  KCNS 

3-75 

m/24  K2SO4 

2.9 

m/24  KCOOCH3 

3-4 

m/24  K2C2O4 

3-4 

Salts  of  the  heavy  metals 

Influence  of  different  cations  with  common 
anion 

1.25  per  cent,  albumin 

1.25  per  cent,  gelatine 

1.25  percent,  albumin 

1.25  per  cent,  albumin 

c 

c 

.H 

.9 

a 

i 

a 

a 

5 

c 

3 

o 

(D 

.2 

• 

o 

I 

S 

I* 

S 

a 

PV«I 
on 

a 

y 

•sw 

0 

Sif 

8 

|fi 

N 

0) 

6 

6s 

3 

6s 

6 

l« 

d 

Is 

0 

21.5 

0 

5-4 

o 

20.8 

o 

5.4 

m/96  MnCl2 

6.9 

m/i92  CoCl2 

2.0 

m/48  LiCl 

5.4 

m/48  LiCl 

2.9 

m/96  CoCl2* 

5-6 

m/i92  CuCl2 

3-3 

m/48  NaCl 

S-6 

m/48  NaCl 

2.6- 

m/96  CdCl2* 

4.1 

m/48  KC1 

5-9 

m/48  KC1 

2.4 

m/96  Pb- 

2.8 

m/48NH4Cl 

4-5 

m/48  NH4C1 

2.6 

(N03)2* 

m/96  CuCl2* 

1.6 

*  A  precipitate  is  formed. 

It  is  interesting  to  compare  the  behavior  of  the  two  colloids 
toward  the  same  added  substance.  While  the  salts  of  the  alkali 
metals  produce  about  the  same  effect  upon  both  (the  sulpho- 


MECHANICAL   PROPERTIES    OF   COLLOID    SYSTEMS 


251 


cyanate  having  the  least  effect,  the  sulphate  the  greatest)  almost 
opposite  effects  are  produced  on  albumin  and  gelatine  when 
other  salts  are  used.  Among  the  alkaline  earths,  SrCl2  and 
MgCl2  produce  a  greater  effect  on  albumin  than  CaCl2  or  BaCl2. 
When  gelatine  is  used  the  reverse  is  the  case.  Of  the  salts  of 
the  heavy  metals,  CuCl2  affects  albumin  more  than  CoCl2.  The 
opposite  is  true  for  gelatine.  Such  contrary  effects  are  not  so 
evident  when  the  cations  are  compared. 

On  the  basis  of  the  investigations  of  S.  Posternak,1  Wo.  Pauli2 


A/a  Acetate 


NaBr 


m/96 


Concentration 


FIG.  58. — Effect  of  salts  upon  the  osmotic  pressure  of  gelatine.     (According 

to  R.  S.  Lillie.) 

and  R.  Hober,3  we  are,  no  doubt,  correct  in  referring  these  dif- 
ferences in  behavior  to  the  differences  in  the  reaction  of  the  two 
colloids.  Fresh  (native)  albumin,  such  as  R.  S.  Lillie  used,  has  a 
slightly  alkaline  reaction,  while  commercial  gelatine  is  always 
acid.  The  differences  in  the  effects  of  an  added  salt  upon  an 
"acid"  or  an  "alkaline"  albumin  so  far  as  its  internal  friction 
was  concerned  was  discussed  in  §25.  It  is  of  much  interest  that 
the  osmotic  pressure  of  colloid  systems  should  also  be  so  greatly 
dependent  on  the  acid  or  alkaline  reaction  of  the  colloid. 

1  S.  Posternak,  Ann.  de  PInst.-Pasteur,  15,  85  (1909). 

2  Wo.  Pauli,  Hofmeister's  Beitr.,  5,  27  (1903). 

3  R.  Hober,  Hofmeister's  Beitr.,  n,  35  (1907). 


252 


SPECIAL   COLLOID-CHEMISTRY 


The  influence  of  electrolytes  on  the  osmotic  pressure  *of  colloids 
may  show  hysteresis.  The  after-effects  of  temperature  were 
discussed  on  p.  243.  If,  in  the  osmotic  study  of  a  gelatine  + 
acid  mixture,  the  outer  liquid  is  replaced  by  distilled  water,  the 
pressure  column  gradually  sinks.  But  to  attain  its  original  level 


ConcenhnaHon 


FIG.  59. — Effect  of  salts  upon  the  osmotic  pressure  of  albumin.     (According 

to  R.  S.  Lillie.) 

requires  days,  and  maybe  weeks,  before  the  osmotic  pressure  of 
the  pure  gelatine  is  again  reached,  even  when  the  acid  which 
dialyzes  out  very  rapidly,  is  constantly  removed  by  frequent 
changes  of  the  water  (R.  S.  Lillie).  Such  lagging  before  equilib- 
rium is  finally  attained  is  unknown  in  the  osmosis  of  molecular 
dispersoids. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS 


253 


Our  knowledge  of  the  influence  of  non-electrolytes  on  the 
osmotic  pressure  of  colloids  is  still  limited.  An  investigation  of 
this  question  would  doubtless  bring  out  many  interesting  facts. 
Table  59  reproduces  some  of  R.  S.  Lillie's  (I.e.)  results,  in  which 
but  small  differences  of  both  a  positive  and  a  negative  nature 
appear.  Obviously,  higher  concentrations  of  alcohol,  acetone, 
etc.,  might  cause  a  decided  decrease  in  the  osmotic  pressure  of 
these  colloids. 

TABLE  59. — INFLUENCE  OF  NON-ELECTROLYTES  ON  OSMOTIC  PRESSURE  OF 

COLLOIDS 
(According  to  R.  S.  Lillie) 

Egg  Albumin 


1  .  25  pe 

r  cent. 

1.6  pe 

r  cent. 

Added  substance 

Osmotic  pressure 
in  mm.,  Hg. 

Added  substance 

Osmotic  pressure 
in  mm.,  Hg. 

o 
m/6  cane  sugar 
m/6  dextrose 

22.4 

21-5 
21.8 

O 

m/6  glycerine 
m/6  urea 

29.4 

29-5 
27.9 

Gelatine  1.25  per  cent. 


o 

6.2                                 o                                5.5 

m/6  cane  sugar 

6.6                    m/6  dextrose 

5-7 

m/6  dextrose 

5-8 

m/6  glycerine 

5-6 

m/6  glycerine 

5-9 

m/6  urea 

6.6 

m/6  urea 

7-3 

7.  On  the  Theory  of  Osmotic  Pressure  of  Colloids. — In  the 

classic  theory  of  osmosis  in  molecularly  dispersed  systems,  as 
formulated  by  J.  H.  van't  Hoff,  on  the  basis  of  W.  Pfeffer's  (I.e.) 
experiments,  the  absolute  concentration,  in  other  words,  the 
number  of  molecules  in  the  unit  volume  alone  determines  the  amount 
of  the  osmotic  pressure  (at  constant  temperature).  The  osmotic 
pressure  is  directly  proportional  to  the  number  of  molecules  and 
to  the  absolute  temperature.  Sv.  Arrhenius  assumed  a  dissocia- 
tion of  the  molecules  into  ions,  in  the  case  of  the  electrolytes  in 
which  a  gram  molecule  in  the  unit  volume  shows  a  higher  osmotic 
pressure  than  that  calculated.  On  the  other  hand,  when  unex- 


254  SPECIAL   COLLOID-CHEMISTRY 

pectedly  low  osmotic  pressures  were  observed,  as  in  high  con- 
centrations of  different  substances,  it  was  held  that  there  occurred 
association,  polymerization,  etc.,  of  single  molecules  to  larger 
aggregates,  or  that  the  dissolved  substances  combined  with  the 
dispersion  means  to  form  solvates,  etc.  But  whatever  the  ir- 
regularities observed,  they  were  uniformly  reduced  to  either  an 
increase  or  a  decrease  in  the  number  of  particles  actually  present 
in  the  unit  volume  as  compared  with  their  calculated  number. 
The  number  of  particles  has,  in  other  words,  in  this  classic  theory 
of  solution,  been  regarded  as  the  most  important  if  not  the  sole 
variable. 

When  the  osmotic  phenomena  of  dispersed  systems  are  viewed 
in  a  more  general  way,  especially  in  connection  with  other  forms 
of  movement,  as  Brownian  movement  and  diffusion,  it  becomes 
evident  that  several  other  variables,  not  considered  in  the  classic 
theory  of  osmosis,  play  an  important  part.  They  are  the  degree 
of  dispersion  and  the  type  of  the  dispersed  phase,  together  with 
such  associated  properties,  as  degree  of  hydration,  etc.  It  makes 
no  difference  in  the  classic  theory  of  osmosis  what  is  the  size  of 
the  dispersed  particles,  or  whether  we  deal  with  molecularly  and 
ionically  dispersed  phases  or  with  coarse  dispersions.  Nor  does 
the  type  of  the  dispersed  phase  matter,  or  its  degree  of  hydration, 
except  in  so  far  as  through  hydration  a  portion  of  the  solvent  may 
be  withdrawn,  thereby  causing  an  increase  in  the  molar  concen- 
tration. With  any  given  substance  in  a  given  dispersion  means, 
each  particle,  no  matter  what  its  type  or  size,  behaves  like  a 
molecule,  and  if  N  particles  (Avogadro's  number)  are  present  in 
the  unit  volume,  the  system  will  exert  unit  osmotic  pressure. 

It  is  evident  that  we  may  not  thus  assume  the  independence  of 
osmotic  pressure,  say  of  the  degree  of  dispersion,  when  we  come  to 
deal  with  systems  which  have  not  a  maximal  degree  of  it,  as  in 
colloid  solutions.  To  do  so  would  be  to  deny  the  importance  of 
the  relations  between  osmosis,  diffusion  and  Brownian  movement. 
We  cannot  ascribe  the  small  pressures  exhibited  by  colloids  to  a 
low  " molar"  concentration  of  the  colloid  phase.  Just  as  certainly 
as  highly  dispersed  phases  possess  a  greater  Brownian  movement 
and  a  higher  diffusion  coefficient,  even  independently  of  their 
concentration,  equally  certainly  must  they  show  a  greater  osmotic 
pressure  than  less  dispersed  ones,  other  conditions  being  equal. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  255 

Only  a  theory  of  osmotic  phenomena  that  considers  the  degree  of 
dispersion  of  a  system,  in  addition  to  concentration  and  tem- 
perature, can  prove  universally  valid  for  all  dispersed  systems. 

It  is  not  difficult  to  bring  experimental  proof  for  such  theo- 
retical deductions.  In  fact,  no  one  who  tries  to  account  for  the 
great  sensitiveness  of  the  osmotic  pressure  of  colloids  to  different 
influences,  can  escape  considering  changes  in  those  characteristic 
variables  of  colloids,  namely,  their  degree  of  dispersion  and  their 
type,  as  responsible  for  it.  The  variations  in  the  osmotic  pressure 
must  be  explained  by  the  same  kind  of  changes  by  which  we 
explain,  for  example,  the  variations  in  their  internal  friction, 
namely,  "  changes  in  state." 

The  influence  of  degree  of  dispersion  upon  osmotic  pressure  is 
very  evident  in  congo  red.  W.  M.  Bayliss  (I.e.)  prepared  a  pure 
and  highly  dispersed  congo  red  by  allowing  NaOH  to  diffuse  into 
its  free  acid  contained  in  an  osmometer.1  While  the  free  acid  is 
pronouncedly  colloid,  as  betrayed  by  the  fact  that  it  is  readily 
analyzable  ultramicroscopically,  congo  red  prepared  in  the  manner 
described,  cannot  be  thus  analyzed.  But  it  can  be  as  soon  as 
traces  of  electrolytes  are  added.  Even  the  carbonic  acid  of  the 
air  suffices  to  do  this.  At  the  same  time,  the  osmotic  pressure 
of  the  system  decreases.  All  factors  which  cause  a  decrease  in 
degree  of  dispersion,  as  the  addition  of  electrolytes,  ageing,  shak- 
ing, etc.,  decrease  the  osmotic  pressure.  Other  factors  which 
increase  the  osmotic  pressure,  as  the  addition  of  alkalies,  also 
make  the  ultramicroscopically  heterogeneous  structure  give  way 
to  an  optically  homogeneous  one. 

The  fact  observed  by  J.  Duclaux2  that  the  osmotic  pressure  of 
a  red  gold  hydrosol  is  considerably  greater  than  that  of  a  blue 
one  also  belongs  here.  We  have  every  reason  for  believing  that 
blue  gold  sols  are  not  as  highly  dispersed  as  red  ones. 

The  view  advanced  here  that  changes  in  the  state  of  a  colloid, 
more  especially  variations  in  its  degree  of  dispersion  and  its  type, 
are  of  particular  significance  in  determining  its  osmotic  pressure, 
is  perhaps  most  clearly  demonstrated  by  the  close  analogies  between 
the  osmotic  phenomena  exhibited  by  colloids  and  their  internal 

1  The  similar  behavior  of  freshly  prepared  silicic  acid  is  discussed  on  p.  227. 

*  J.  Duclaux,  Compt.  rend.,  148,  295  (1909);  for  a  description  of  the  special  method 
used  by  this  author  in  determining  the  osmotic  pressure  see  this  paper  and  Koll.- 
Zeitschr.,  3,  134  (1908). 


256  SPECIAL   COLLOID-CHEMISTRY 

friction  and  swelling.  The  close  relationship  between  these  pro- 
cesses is  brought  out  not  only  by  emphasizing  that  age,  previous 
thermal  history  and  mechanical  treatment  affect  all  of  them  in 
the  same  general  way,  but  by  the  fact  that  they  do  this  often 
down  to  the  minutest  details.  This  is  clearly  apparent  when 
we  compare  the  influence  of  acids  and  alkalies  on  the  osmotic 
pressure  (R.  S.  Lillie)  with  their  effect  upon  the  internal  friction 
(Wo.  Pauli,  etc.).  Still  more  striking,  perhaps,  is  a  comparison 
of  the  effects  of  acids  and  alkalies  on  the  osmotic  pressure  of  1.25 
per  cent,  gelatine  solutions  (R.  S.  Lillie)  with  those  of  these 
same  substances  on  the  swelling  of  gelatine  discs  (Wo.  Ostwald).1 
Here  the  agreement  is  perfect  even  to  details  (see  Figs.  55,  56, 
pp.  246,  247). 2  In  connection  with  these  facts  the  influence  of 
added  substances  on  the  viscosity  of  gelatine  solutions,  as  given  on 
p.  169,  should  also  be  studied. 

As  a  matter  of  fact,  the  relation  between  osmosis  and  swelling 
is  close  even  when  the  question  is  viewed  from  a  theoretical 
standpoint.  In  the  place  of  a  selectively  permeable  membrane, 
we  have  the  structure  of  the  material  undergoing  swelling  which 
hinders  the  movement  of  the  dispersed  phase  into  the  dispersion 
or  swelling  means.  The  process  leading  to  the  highest  attainable 
homogeneous  (spatial)  distribution  of  swelling  substance  and 
swelling  producing  medium,  is  possible  only  if  the  structure  and 
the  specific  surface  of  the  swelling  body  change  simultaneously, 
while  the  spatial  relationship  of  the  two  phases  to  each  other 
remains.  If  this  relationship  is  destroyed,  as  by  increase  of 
temperature  above  a  critical  value,  then  instead  of  swelling, 
solution  occurs.  Besides  these  analogies  between  the  osmosis 
and  the  swelling  of  colloids  (as  well  as  between  osmotic  and 
swelling  pressures),  characteristic  differences  also  exist  between 
them.  In  the  process  of  swelling,  a  radical  change  in  state,  namely, 
an  increase  in  degree  of  dispersion  takes  place.  In  the  osmotic 
processes  of  molecularly  dispersed  systems,  the  specific  surfaces, 

1  Wo.  Ostwald,  Pfliiger's  Arch.,  108,  563  (1905). 

2  According  to  R.  S.  Lillie  the  acid  minimum  is  about  one-tenth  that  found  by 
Wo.  Ostwald  in  his  experiments  on  swelling.     But  since  the  latter  minimum  is 
practically  identical  with  that  of  the  viscosity  maximum  of  dilute  gelatine  solutions 
as  found  by  P.  von  Schroeder  (p.  169)  and  agrees  fully  with  the  acid  maximum 
for  albumin  solutions  (see  H.  Handovsky,  Koll.-Zeitschr.,  7,  192,  1910)  Lillie's  figure 
evidently  represents  an  error  either  in  measurement  or  calculation. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  257 

etc.,  of  the  dispersed  particles  remain  constant  and  only  the  re- 
lation of  number  of  particles  to  unit  volume  changes.  But  when 
colloid  systems  are  under  discussion  the  processes  of  swelling 
and  osmosis  again  agree;  in  fact,  the  osmosis  of  colloid  solutions 
might  well  be  termed  a  "swelling  of  liquids"  in  contrast  to  the 
usual  swelling  of  solids. 

In  considering  the  enormous  effect  of  acids  and  alkalies  on  the 
osmotic  pressure  of  colloids  one  might  try  to  save  the  classic  con- 
ception of  osmosis  by  assuming  an  increase  in  the  molecular  con- 
centration of  the  albumin  particles,  caused,  say,  by  hydrolytic 
cleavage.  But  examination  of  this  idea  leads  to  an  exactly 
opposite  conclusion,  for,  as  Wo.  Pauli1  has  emphasized,  and  as 
St.  Burgarsky  and  L.  Liebermann  first  showed,  the  observed 
freezing  points  of  mixtures  of  acid  and  alkali  with  albumin  are 
not  as  low  as  those  obtained  by  adding  together  the  effects  which 
albumin  and  the  added  substance  produce  alone.  A  decrease  in 
the  molar  concentration  therefore  occurs,  either  by  chemical  or 
adsorptive  union  of  albumin  with  electrolytes.  The  addition  of 
acids  and  alkalies  as  emphasized  in  the  discussion  of  viscosity  on 
p.  173  leads  to  the  formation  of  a  larger  number  of  albumin 
ions  which  are  capable  of  holding  more  water  than  the  neutral 
albumin  particles.  The  emulsoid  properties  of  the  system, 
originally  relatively  low,  are,  therefore,  greatly  increased,  as  be- 
trayed, for  example,  by  the  rise  in  its  internal  friction,  indiffer- 
ence toward  salt,  etc. 

The  remarkable  effects  of  concentration  and  of  temperature 
on  the  osmotic  pressure  of  a  colloid  will  some  day,  no  doubt,  be 
similarly  explained  through  the  changes  in  the  state  of  the  colloid 
produced  by  them.  It  need  but  be  recalled  that  the  degree  of 
dispersion  and  the  type  of  the  dispersed  phase  are,  at  times, 
a  function  of  the  concentration  and  the  temperature  as  discussed 
on  p.  35.  When  the  degree  of  dispersion  decreases  with  rise  in 
concentration,  as  in  soap  solutions,  then  the  (relative)  osmotic 
pressure  must  decrease.  Actually  this  is  found  to  be  true  not 
only  for  soap  solutions  but  also  for  hemoglobin  (see  Table  52 
on  p.  241).  Analogous  considerations  hold  for  the  effects  of 
temperature  on  the  osmotic  pressure  of  different  colloid  systems. 

1  Wo.  Pauli,  Pfliiger's  Arch.  (1910).    Festschr.  f.  E.  Hering.    Prof.  Pauli  was  kind 
enough  to  place  the  proof  sheets  of  this  article  at  my  disposal. 

17 


258  SPECIAL    COLLOID-CHEMISTRY 

The  many  and  complicated  possibilities  for  great  variations  in 
behavior,  especially  among  the  emulsoids  belonging  to  the  number 
of  the  complex  dispersoids,  may  be  foreseen,  especially  when  the 
additional  variations  which  may  be  introduced  through  changes 
in  the  electrical  properties  are  kept  in  mind.  The  suspensoids, 
which  assume  but  one  form,  will  show  a  simpler  behavior.  That 
this  is  so  is  borne  out  by  the  observations  on  dyes  of  the  suspen- 
soid  type,  as  congo  red,  benzopurpurin,  etc.,  as  studied  by  W.  M. 
Bayliss  (I.e.),  W.  Biltz,  A.  von  Vegesack  (I.e.)  and  others.  The 
problem  of  the  future  is  more  the  problem  of  analyzing  the  type 
of  these  various  colloid  changes  than  that  of  settling  whether  or 
not  the  observed  peculiarities  can  be  explained  on  the  basis  of  the 
classic  theory  of  osmosis. 

In  a  word,  then,  the  osmotic  pressure  of  most  colloids  is 
by  no  means  only  a  function  of  the  number  of  particles  in  the  unit 
volume,  but  varies  with  the  changes  in  the  state  of  these  sys- 
tems, more  especially  with  the  changes  in  the  degree  of  dispersion 
and  the  type  of  the  dispersed  phase.  The  value  of  the  osmotic 
pressure  is  therefore  a  more  complex  function  in  the  case  of 
colloids  than  in  molecularly  dispersed  systems,  and  may  not  off- 
hand be  made  identical  with  the  latter.  In  fact  it  seems  im- 
possible, for  these  reasons,  to  assign  absolute  values  to  the  osmotic 
pressure  of  colloidally  dispersed  systems.  This  is  true  of  all 
emulsoid  and  complex  dispersoids,  while  simpler  relations,  re- 
sembling those  valid  for  the  molecularly  dispersed  systems,  seem 
to  exist  in  the  case  of  suspensoid  systems  (see  the  succeeding 
paragraphs).  Perhaps  future  investigators  will  find  it  best  to 
reserve  the  concept  of  osmosis  for  molecular  dispersoids  and  to 
use  another  term  like  hydration  (solvation)  for  the  phenomena 
observed  in  colloid  and  coarsely  dispersed  systems.  Such  a  term 
would  constantly  bring  to  mind  the  important  difference  be- 
tween the  two  kinds  of  phenomena. 

8.  Determination  of  the  "Molecular  Weight"  of  Colloid  Sys- 
tems by  Osmotic  Means.  —  As  is  well  known,  the  molecular  weight 
of  a  dissolved  substance  may  be  determined  from  the  osmotic  pres- 
sure of  a  molecularly  dispersed  solution,  by  the  following  formula  : 


. 

M  =  (22.4  X  760)-  •„, 
P-J-  o 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  259 

in  which  M  represents  the  molecular  weight  sought,  22.4  the 
" normal"  osmotic  pressure  of  a  gram-molecule  of  the  molecularly 
dispersed  substance  at  o°,  c  the  concentration  (in  per  cent.),  p 
the  observed  osmotic  pressure  in  mm.  of  Hg.,  TI,  the  observed 
absolute  temperature,  and  TQ,  273°.  Since  J.  H.  van't  Hoff 
first  formulated  this  law  the  different  investigators  who  have 
measured  the  osmotic  pressure  of  colloids  have,  also,  in  good 
part  tried  to  deduce  therefrom  their  "molecular  weight."  Indeed, 
there  exist  but  few  publications  on  the  osmotic  pressure  of  colloids 
in  which  there  is  not  a  column  devoted  to  their  "  molecular  weight" 
as  calculated  from  the  osmotic  pressures.  Many  examples  could 
be  given  of  this. 

The  striking  thing  about  these  "molecular  weights"  of  colloid 
systems  is  their  great  absolute  value  and  their  great  variability  under 
different  conditions.  The  former  seems  obvious  enough  in  view 
of  the  low  values  found  for  the  osmotic  pressure  of  colloids  of 
even  simple  chemical  composition.  The  second,  however,  accord- 
ing to  which  the  molecular  weight  varies  under  different  cir- 
cumstances is,  strictly  speaking,  a  contradiction  in  terms,  for  by 
definition,  the  molecular  weight  is  a  constant.  But  disparities 
between  the  molecular  weights  of  substances  as  deduced  from 
osmotic  measurements  and  from  analysis,  have  been  observed 
in  molecularly  dispersed  systems  also.  In  other  words,  the  simple 
proportion  between  osmotic  pressure  and  concentration,  as 
demanded  by  theory,  has  not  always  been  observed  to  hold  even 
here.  Thus  W.  M.  Bayliss  (I.e.,  1910)  cites  the  fact  that  the 
molecular  weight  of  alcohol  dissolved  in  benzene  rises  from  50 
to  208,  is  quadrupled,  in  other  words,  in  passing  from  a  con- 
centration of  0.494  per  cent,  to  one  of  14.63  per  cent.  It  must  be 
left  to  the  students  of  the  molecularly  dispersed  solutions  to  inter- 
pret these  contradictions  between  their  fundamental  equation  and 
its  applications.  But  so  far  as  the  colloids  are  concerned,  such  a 
calculation  of  molecular  weight  from  osmotic  measurements  can 
never  be  attempted  with  safety  because  it  is  wrong  in  principle. 
Not  even  the  sense  of  the  variations  in  the  osmotic  pressure  of  colloid 
solutions  and  their  concentration  need  be  the  same  in  all  cases.  On 
p.  240  it  was  pointed  out  that,  according  to  J.  Duclaux,  the 
osmotic  pressure  of  iron  hydroxidesol  increases  more  rapidly  than 
its  concentration.  Thus,  while  we  generally  observe  an  increase 


260  SPECIAL    COLLOID-CHEMISTRY 

in  the  molecular  weight  with  an  increase  in  concentration  as 
expressed  by  the  relatively  smaller  increase  in  the  osmotic  pres- 
sure, in  the  example  just  cited  we  would  be  dealing  with  a  decrease 
in  molecular  weight,  even  to  one-tenth  the  original.  The  "  mo- 
lecular weights"  of  acid  and  alkali  albumin  would  even  be  found 
to  yield  complicated  curves  with  maxima  and  minima  related 
to  the  concentration  of  the  added  electrolytes.  In  fact,  two 
or  three  entirely  different  concentrations  of  acid  or  alkali  would 
be  found  in  which  the  molecular  weights  of  the  albumin,  or  its 
combination  with  an  electrolyte,  would  be  the  same,  thereby 
contrasting  with  the  molecular  weights  observed  in  all  other 
concentrations.  Similarly,  salts  would  affect  the  "molecular 
weight, "  making  it  either  rise  or  fall,  depending  solely  upon  the 
concentration  of  the  added  electrolyte.  Depending  upon  the 
acid  or  alkaline  reaction  of  the  colloid,  the  "molecular  weight" 
of  a  colloid  might  be  either  raised  or  lowered  on  adding  a  salt. 
With  rising  temperature,  the  molecular  weight  of  some  colloids 
would  be  increased,  of  others  decreased.  The  "molecular 
weight"  of  a  colloid  would  be  changed  by  shaking  or  stirring,  by 
ageing  and  by  being  warmed  either  slowly  or  rapidly.  These 
illustrations  will  suffice  to  demonstrate  the  impropriety  of  apply- 
ing the  ordinary  concept  of  "molecular  weight"  to  colloid  solu- 
tions.1 It  is  hard  to  see  how  a  "constant"  which  varies  between 
several  hundred  and  infinity  with  concentration  alone,  as  in 
soap  solutions,  can  be  of  any  value  in  the  physico-chemical  char- 
acterization of  a  system. 

In  this  condemnation  of  the  value  of  "molecular  weight'' 
determinations  of  colloidally  dissolved  substances  by  osmotic 
methods,2  it  is  not  maintained  that  there  may  not  exist  transition 
systems  between  colloidally  and  molecularly  dispersed  systems  in 
which  there  is  at  least  an  approximate  proportionality  between 
osmotic  pressure  and  concentration,  and  therefore  a  proper  basis 
for  the  calculation  of  the  molecular  weight.  In  fact,  W.  M. 
Bayliss  (I.e.)  discovered  such  a  system  in  congo  red,  freshly  pre- 
pared by  the  method  described  above  (see  also  W.  Biltz  and 
A.  von  Vegesack,  I.e.).  This  dye  when  fresh  and  free  from 
electrolytes,  is  highly  dispersed,  as  evidenced  by  its  ultramicro- 

1  See  in  this  connection  J.  Duclaux,  Compt.  rend.,  148,  714  (1909). 

2  For  the  determination  of  the  "  molecular  weight "  of  colloids  by  indirect  methods, 
as  by  measuring  the  vapor  tension,  the  boiling  or  freezing  points,  etc.,  see  p.  142. 


MECHANICAL   PROPERTIES    OF    COLLOID    SYSTEMS  261 

scopic  properties,  its  considerable  osmotic  pressure,  etc.  In 
this  pure  condition  a  0.465  per  cent,  solution  yields  an  osmotic 
pressure  of  60  mm.  of  water.  If  now,  by  the  formula  given  above, 
the  moleqular  weight  of  the  pure  congo  red  is  calculated,  the 
answer  is  a  value  of  90  to  95  per  cent,  of  that  obtained  by  analytical 
methods  (696.47).  W.  Biltz  and  F.  Pfenning  obtained  similar 
results.  This  shows  that  pure  congo  red  behaves  like  a  typical 
molecular  dispersoid,  at  least  in  its  osmotic  relations.  The 
applicability  of  the  above  formula  to  the  determination  of  the 
molecular  weight  of  this  dye  is  also  evidenced  by  the  direct 
proportionality  existing  between  concentration  and  pressure,  in 

P 

other  words,  the  constancy  of  the   quotient  -7^  as  evidenced  in 

Table  49,  on  p.  239.  In  cases  of  this  type,  and  only  in  such, 
are  molecular  weight  determinations  by  this  method  justified. 
Moreover,  the  considerable  electrical  conductivity  of  pure  congo 
red  solutions  as  studied  by  W.  Biltz  and  A.  von  Vegesack  further 
shows  that  we  deal  in  this  case  with  a  molecular  dispersoid  rather 
than  with  a  colloid,  for  high  conductivity  is  not  characteristic  of 
typical  colloids. 

9.  On  the  Moleculo-kinetic  Theory  of  Osmosis  in  Colloid 
Systems. — In  view  of  the  successful  applications  that  have  been 
made  of  moleculo-kinetic  conceptions  to  the  quantitative  study 
of  the  phenomena  of  movement  exhibited  in  colloid  systems,  it 
may  be  asked  if  they  may  not  also  be  of  service  in  the  theory  of 
the  osmotic  pressure  of  these  systems.  A.  Einstein  and  M.  von 
Smoluchowski1  have  considered  this  question.  They  conclude 
that  the  osmotic  pressures  of  two  equally  concentrated  but  differently 
dispersed  phases  are  inversely  proportional  to  the  cubes  of  the  radii, 
of  their  particles.'1  In  other  words, 

PI    M3 
p2     (ri)' 

This  highly  interesting  conclusion  has  not  yet  been  tested 
experimentally. 

It  is  of  interest  that  the  above  conclusion  was  reached  on  the 
basis  of  considerations  in  which  it  was  assumed  that  the  Boyle- 
Gay-Lussac  law  (direct  proportion  between  pressure  and  con- 

1  M.  von  Smoluchowski,  Boltzmann-Festschrift,  626,  Leipzig,  1904. 

2  See  The  Svedberg,  van  Bemmelen-Gedenkboek,  131,  1910. 


262  SPECIAL   COLLOID  CHEMISTRY 

centration  as  well  as  absolute  temperature)  was  valid.  This 
assumption  holds,  of  course,  only  at  great  dilutions.  The  Sved- 
berg1  tried  to  determine  indirectly  the  validity  of  the  Boyle- Gay- 
Lussac  law  for  colloids.  An  equation  governing  local  changes  in 
the  motion  of  particles  showing  Brownian  movement,  so  far  as  extent 
and  frequency  are  concerned,  may  be  derived  from  the  equation  of 
von  Smoluchowski  (I.e.).  A  detailed  exposition  of  this  second 
equation  and  the  considerations  leading  to  it  cannot  be  given  here. 
The  Svedberg,  however,  found  highly  diluted  gold  and  mercury 
sols  to  obey  it.  He  concluded,  therefore,  that  the  Boyle-Gay- 
Lussac  law  used  in  constructing  the  formula  would  also  have  to  be 
valid  for  greatly  diluted  colloid  systems.  It  is  perhaps  too  early 
to  concur  entirely  in  this  conclusion,  since  the  number  of  mathe- 
matical assumptions  in  the  formula  is  exceedingly  great.  Be- 
sides, Svedberg's  figures  (see  especially  their  graphic  represen- 
tation on  p.  555  of  his  paper,  I.e.,  1910)  themselves  show  that 
the  law  holds  strictly  only  at  a  transition  point,  for  only  in  very 
dilute  concentrations  is  there  strict  agreement  between  observa- 
tion and  theory.  Deviations  from  the  rule,  and  therefore  from 
the  Boyle- Gay-Lussac  law,  begin  to  appear  in  the  case  of  a 
mercury  sol  as  soon  as  its  concentration  amounts  to  i/6.io~10 
normal,  or  about  0.000,000,000,3  per  cent,  by  weight.  In  view  of 
the  slight  practical  significance  of  the  concentration  range  over 
which  it  is  valid,  the  law  appears  to  be  an  ingenious  theoretical 
deduction  more  than  a  means  of  studying  quantitatively  the  de- 
pendence of  osmotic  pressure  in  colloid  systems  on  concentration 
and  temperature. 

Addendum :  Other  Types  of  Movement  in  Dispersoids 

The  phenomena  of  movement  observed  in  colloid  systems 
under  the  influence  of  an  electric  current  will  be  discussed  later. 
At  this  point  we  merely  wish  to  mention  the  phenomena  of  move- 
ment which  occur  under  the  directive  influence  of  heat  and  light. 
The  botanists  F.  Stahl2  and  W.  Sachs3  observed  such  directed 
movements  in  small  solid  and  liquid  particles  while  attempting  in 
1876  to  determine  to  what  extent  the  thermal  and  helio tropic  move- 
ments of  unicellular  organisms  (such  as  zoospores)  depended  upon 

JThe  Svedberg  (I.e.},  as  well  as  Zeitschr.  f.  physik.  Chem.,  73,  547  (191°)- 

2  F.  Stahl,  Bot.  Ztg.,  715  (1876);  Verb.  d.  phys.-med.  Ges.  Wurzburg,  14  (1879). 

3  W.  Sachs,  Flora,  241  (1876).     See  also  E.  Strassburger,  Jenaisch.  Z.  f.  Naturw., 
12  (1878). 


MECHANICAL   PROPERTIES    OF    COLLOID   SYSTEMS 


263 


biological 'properties  and  in  how  far  they  were  merely  passive. 
The  directive  influence  of  light  on  the  movement  of  dispersed 
particles  was  later  studied  in  detail  by  G.  Quincke.1  W.  R. 
Whitney  and  C.  J.  Blake2  have  studied  such  light  and  heat  effects 
on  the  movement  of  colloid  particles  in  colloid  gold.  The  directive 
influence  of  light  on  crystallization  and  sublimation  should  also  be 
mentioned  here.3 

§31.  Filtration  and  Ultrafiltration  of  Colloid  Systems 

i.  Filtration  of  Colloid  Systems. — A  property  which  dis- 
tinguishes colloid  solutions  from  coarse  suspensions  is  the  ability 
of  the  former  to  pass  unchanged  through  filter  paper.  It  is  by 
this  means  that  we  recognize  the  formation  of  a  colloid  solution 
when  we  wash  a  precipitate  with  pure  water.  While  typical 
colloids  pass  through  all  filter  papers,  somewhat  coarser  systems 
begin  to  be  held  back  by  hardened  filter  papers  and  by  clay  and 
porcelain  filters  as  those  of  Berkefeld,  Reichel,  Chamberland  and 
Pukall. 

The  filtrability  of  a  dispersoid  depends  upon  the  size,  shape  and 
rigidity  of  its  particles,  upon  the  filtration  pressure  and  the  nature 
of  the  filter,  more  especially  the  size  of  its  pores.4  To  determine 
the  approximate  size  of  the  dispersed  particles  it  is  therefore  well 
to  know  the  average  size  of  the  pores  of  different  filters.  Such 
determinations,  as  made  by  H.  Bechhold,5  are  given  in  Table  60. 

TABLE  60. — SIZE  OF  PORES  IN  FILTERS 
(According  to  H.  Bechhold) 


Filter 

Average  size  of 
pores  (permeability 
to  water) 

Size  of  largest 
pores  (permeability 
to  air) 

Ordinary  thick  filter  paper  .... 

2      2i( 

Filter  paper  No.  556  (Schleicher  and  Schiill).  . 
Filter  paper  No.  602  (extra  hard,  Schleicher 
and  Schull). 
Chamberland  filter  

I.7M 
0.89-1.3;* 

l-I.S/z 
0.23-0.41/4 

Reichel  filter  

0.16—  o.  i7S/i 

1  G.  Quincke,  Report  Brit.  Assoc.  Advanc.  Science,  Glasgow,  60  (1901);  Drude's 
Ann.  d.  Physik.,  7,  701  (902). 

2  W.  R.  Whitney  and  J.  C.  Blake,  Journ.  Amer.  Chem.  Soc.,  26,  1347  (1904). 

3  See  the  summary  of  J.  M.  Eder,  Photochemie,  3  Aufl.,  123,  Halle,  1906. 

4  Details  regarding  this  question  may  be  found  in  the  paper  of  E.  Hatschek, 
J.  Soc.  Chem.  Industry,  27,  538  (1908);  also  Koll.-Zeitschr.,  6,  254  (1910);  7,  81 
(1910). 

5  H.  Bechhold,  Zeitschr.  f.  physik.  Chem.,  64,  328  (198). 


264  SPECIAL    COLLOID-CHEMISTRY 

The  original  paper  must  be  consulted  for  details  of  the  methods 
used  by  Bechhold  in  arriving  at  the  assigned  values. 

As  the  table  shows,  typical  colloids,  with  particles  having  a 
diameter  of  less  than  o.iju,  must  be  just  able  to  pass  through  the 
filter  lowest  in  the  list.  But  even  with  pores  of  this  size  by- 
effects,  known  as  " adsorption"  effects,  often  appear,  due  to  the 
action  of  the  filter  itself  upon  the  dispersed  phase.  These  lead 
to  retention  of  the  dispersed  phase  and  so  to  a  clogging  of  the 
pores  of  the  filter.  At  other  times  coagulation  processes  occur 
due  to  this  surface  action  of  the  filter.  Whenever  any  of  these 
things  take  place,  filtration  cannot,  of  course,  any  longer  tell  us 
anything  definitely  regarding  the  size  of  particles  in  a  dispersoid. 

2.  UltrafHtration  of  Colloid  Systems.— After  W.  Schmidt1  and 
F.  Hoppe-Seyler2  found  that  solutions  of  albumin  and  of  gum 
became  more  dilute  by  being  filtered  through  animal  membranes, 
C.  J.  Martin3  discovered  that  colloidally  dissolved  materials  could 
be  completely  separated  from  their  dispersion  means  by  being 
filtered  through  organic  or  inorganic  gels.  To  give  these  a  proper 
support  he  used  Chamberland  filters  and  impregnated  them 
with  gelatine  or  silicic  acid.  He  could  then  filter  liquids  under  30 
to  100  atmospheres  of  air  pressure  without  breaking  the  filter. 
By  using  this  method,  he  was  able  to  separate  from  the  albumin 
a  clear  fluid  containing  salt  but  entirely  free  of  protein.  Table 
6 1  contains  the  more  important  of  his  results. 

Such  filtration  through  gels  was  next  used  by  French  investi- 
gators (Borrel  and  Manea,  1904;  G.  Malfitano,  1904;  J.  Duclaux, 
I905)4  to  separate  the  dispersed  phase  from  the  dispersion  means 
in  different  organic  and  inorganic  colloids.  They  usually  em- 
ployed collodian  capsules  as  filters. 

H.  Bechhold5  took  an  important  step  forward  in  this  problem 
of  filtration  when  in  1906  he  discovered  the  permeability  of  gels 
to  be  a  function  of  their  concentration.  He  found  dilute  gels  to  be 

1  W.  Schmidt,  Poggendorf's  Ann.,  337  (1856). 

2  F.  Hoppe-Seyler,  Virchow's  Arch.,  9,  245  (1861). 

3C.  J.  Martin,  Journ.  Physiol.,  20,  364  (1896);  see  also  E.  W.  Reid,  ibid  ,  27, 
161  (1903);  A.  Craw,  Zeitschr.  f.  physik.  Chem.,  52,  569  (1898);  Proc.  Roy.  Soc., 
77, 172,  311  (1899). 

4  For  the  history  of  nitration  through  gels  see  J.  Duclaux,  Koll.-Zeitschr.,  3, 
134  (1008);  also  H.  Bechhold,  ibid.,  3,  226  (1908). 

6H.  Bechhold,  Z.  f.  Elektroch.,  12,  777  (1906);  Koll.-Zeitschr.,  I,  107  (1906); 
2,  3  (1907);  Zeitschr.  f.  physik.  Chem.,  60,  237  (1907);  64,  328  (1908). 


MECHANICAL  PROPERTIES  OF  COLLOID  SYSTEMS 


265 


more  permeable  than  more  concentrated  ones.     For  details  re- 
garding his  methods  his  original  publications  must  be  consulted . 

TABLE  61. — FILTRATION  THROUGH  CLAY  CELLS  IMPREGNATED  WITH  SILICIC  ACID 
(According  to  C.  J.  Martin) 


Impermeable  to 


Partially  permeable  to 


Readily  permeable  to 


Egg  albumin. 
Serum  albumin. 
Egg  globulin. 
Serum  globulin. 
Fibrinogen. 
Caseinogen. 
Nucleoalbumin. 
Hemoglobin. 

Glycogen. 

Soluble  starch. 

Soluble  starch  (amylodextrin) 


Alkali  albumin.  i  All  albumoses. 

Acid  albumin.  Urochrome    (pigment    of 

Caramel.  j    urine). 

Biliverdin  (bile  pigment) .  j  All  crystalloids. 

Dextrin. 


TABLE  62. — ULTRAFILTRATION 
(According  to  H.  Bechhold) 


Dispersoid 


Platinum  sol  (Bredig) 

Colloid  iron  hydroxide 

Casein  (of  milk) 

Colloid  gold  containing  so- 
dium lysalbinate  (Zsig- 
mondy). 

Collargol  (v.  Heiden) 

i  per  cent,  hemoglobin  solu- 
tion. 

i  per  cent,  gelatine  solution.. . 

Serum  albumin . . 


The  disperse  phase  is  held 

back  by  a  gelatine  gel  of  the 

following  concentration  in 

per  cent. 


2 

2-5 

3 


3-5 
4 

4 
4-4-5 


Protalbumoses. 

Silicic  acid 

Deutero-albumoses  A 

Deutero-albumoses  B  and  C. 
Dextrin. . 


Remarks 


Average  size  of  particles 
about  44n/j.  (Zsigmondy) . 


About  4Ofj,/ji. 


About 


Molecular  weight — 15,000 
down  to  3000. 


All  crystalloids. 


4-5 

8 

10 

TO 


Molecular    weight    about 

2400. 

|  Traces  pass  through. 
j  Small    amounts    pass 

through;      molecular 

weight  about  965. 
I  Pass  through. 


266 


SPECIAL   COLLOID -CHEMISTRY 


As  a  rule,  he  used  ordinary  filter  papers  as  a  foundation,  impreg- 
nating them  with  various  gels,  as  acetic  acid-collodion,  gelatine- 
formaldehyde,  etc.  Table  62  gives  a  survey  of  his  results. 

It  is  evident  that  the  filter  becomes  less  permeable  as  the 
concentration  of  the  gel  rises.  A  hardened  10  per  cent,  gelatine 
filter  holds  back  even  molecules  of  the  size  of  those  contained  in 
dextrin.  A  proper  series  of  filters  makes  it  possible  to  distinguish, 
within  the  realm  of  the  colloids,  between  systems  of  different  de- 
grees of  dispersion,  and  these  are  then  found  to  correspond  with  a 

differentiation  between  them  made 
on  optical  grounds.  For  this  reason 
H.  Bechhold  has  named  his  method 
u  Ultrafiltration^ 

Recently  A.  Schoep1  has  described 
a  simple  method  of  ultrafiltration, 
in  which  is  eliminated  the  disadvan- 
tage of  having  to  work  with  high 
pressures.2  He  found  that  filters  of 
different  degrees  of  permeability 
could  easily  be  made  by  adding  to 
collodion  solutions  different  amounts 
of  glycerine  and  castor  oil.  Dia- 
lyzing  capsules  may  be  made  from 
such  mixtures  by  the  methods  de- 
scribed in  the  practical  introduction 
on  p.  10.  The  dry  collodion  cap- 
sules become  progressively  more  permeable  (within  certain  limits) 
as  the  amount  of  glycerine  or  castor  oil  in  them  is  increased. 
Fig.  60  illustrates  Schoep' s  simple  method. 

We  cannot  advantageously  discuss  the  theory  of  this  variable 
permeability  of  gels  of  different  concentrations  until  we  have 
taken  up  their  general  structure. 

In  conclusion,  it  must  be  mentioned  that  undesirable  by- 
effects,  such  as  adsorption  of  the  disperse  phase  by  the  filter,  occur 
in  ultrafiltration,  also.  Ultrafiltration  yields  dependable  results, 
therefore,  only  if  checked  up  by  other  methods. 

1  A.  Schoep,  Bull.  Soc.  Chim.  Belg.,  24,  354  (1910);  Koll.-Zeitschr.,  8  (1911).     See 
also  A.  von  Lebedew,  Zentralbl.  f.  Physiol.,  23,  767;  24,511  (1910). 

2  Emulsoids  may  be  separated  from  their  dispersion  means,  with  Schoep's  filter, 
only  when  pressure  is  used. 


FIG.  60. — A .  Schoep's  arrangement 
for  ultrafiltration. 


AUTHOR  INDEX 


Adamson,  L.,  232,  233 

Albanese,  V.,  153 

Alexander,  J.,  no 

Alexandrow,  N.,  131,  142 

Allen,  74 

Amagat,  E.  H.,  117,  120 

Amann,  J.,  i,  60,  69,  227 

Ambronn,  H.,  65 

Antonow,  G.  N.,  184 

Arrhenius,  S.,  94,  141,  214,  218,  253 

Avogadro,  254 

Axelrod,  S.,  153 


B 


Bachmetjew,  Z.,  92 

Barus,  C.,  118,  119,  120 

Baumhauer,  H.,  98 

Bayliss,  W.  M.,  137,  233,  239,  242,  245, 
249,  255,  258,  259,  260 

Bechhold,  H.,  5,  263,  264,  265,  266 

Beck,  K.,  176,  177,  178 

Beer,  i 

Behrens,  H.,  62 

Beibl,  191 

Bemmelen,  J.  M.  van,  23,  87,  88,  89,  106, 
124,  130,  136 

Berkefeld,  263 

Berthelot,  106 

Berzelius,  J.,  52 

Beyerinck,  M.  W.,  138,  177 

Bigelow,  L.,  ii 

Bigland,  D.,  233 

Biltz,  W.,  n,  140,  154,  155,  156,  158, 
159,  163,  164,  165,  176,  225,  227, 
233,  235,  237,  238,  239,  240,  241, 
242,  243,  249,  258,  260,  261 

Blake,  J.  C.,  131,  135,  220,  221,  263 

Bodaszewski,  L.  J.,  191 

Bodenstein,  M.,  94 

Bodlaender,  94 

Borrel,  264 


Bose,  M.,  177 

Bottazzi,  F.,  131, 147, 149, 150, 154, 169, 

181,  184 

Bousfield,  W.  R.,  205 
Boyle,  261,  262 
Brauer,  157 

Bredig,  G.,  24,  90,  93,  94,  135 
Broglie,  M.  de,  191,  193 
Brown,  H.  T.,  131,  132 
Brown,  R.,  186 
Bruni,  G.,  130 
Bruyn,  Lobry  de,  27,  139 
Bugarsky,  St.,  131,  142,  257 
Buglia,  G.,  176,  177,  181,  184 
Burnett,  Th.  C.,  131,  141,  142 
Biitschli,  O.,  62,  106 
Buxton,  B.  H.,  224,  225 


Calcar,  R.  P.  von,  223 
Cavazzani,  E.,  153 
Chamberland,  263 
Chaudesaigues,  P.,  193,  194,  201 
Cholodny,  P.  J.,  121,  135 
Cohen,  E.,  94 
Cotton,  A.,  n,  58,  64 
Coudres,  Th.  Des.,  92 
Craw,  A.,  264 
Curie,  P.,  90 


Dabrowski,  201 

Daguin,  106 

D'Errico,  G.,  131,  147,  149*  *5 

Denning,  Du  Pre,  153 

Doelter,  C.,  57 

Doerinckel,  Fr.,  139 

Donau,  J.,  76,  135 

Donnan,  F.  G.,  68,  83,  84,  177 

Drucker,  K.,  54,  75,  112,  145 

Drude,  P.,  77 


267 


268 


AUTHOR   INDEX 


Duclaux,  J.,  ii,  131,  137,  205,  233,  235, 
239,  240,  241,  242,  243,  255,  259, 
260,  264 

Diillberg,  P.,  131 

Du  Pre  Denning,  153 


Ebbinghaus,  K.,  176,  177 

Eder,  J.  M.,  263 

Eduardoff,  F.,  140 

Ehrenhaft,  F.,  43,  191 

Einstein,  A.,  206,  208,  209,  216,  217,  218, 

219,  261 

Engelmann,  W.,  64 
Erb,  W.,  56 

Errico,  G.  d',  131,  147,  149,  150 
Exner,  F.  M.,  192,  196,  198 
Exner,  S.,  213,  217,  218 


Fano,  G.,  153 

Faraday,  M.,  52 

Fichter,  F.,  15 

Pick,  A.,  210 

Fischer,  M.  H.,  94 

Flemming,  W.,  153,  159 

Frankenheim,  M.  L.,  57,  62,  177 

Free,  E.  E.,  14 

Frei,  W.,  153,  181,  184 

Freundlich,  H.,  50,  55,  67,  68,  94,  100, 

112,  174,  183,  225 
Frey,  W.,  173 
Friedenthal,  H.,  131,  132 
Friedlander,  J.,  7,  55,  56,  105,  147,  177, 

179 
Fuchs,  C.,  68,  190 


Galdi,  F.,  154 

Galeotti,  G.,  106,  174 

Gansser,  233 

Garrett,  H.,  139,  153,  158 

Gatin-Gruszewska,  Z.,    131,    132,    142, 

J43,  147 

Gay-Lussac,  242 
Geffcken,  G.,  136 
Genthe,  A.,  146,  148 


Giampalmo,  G.,  174 

Gibbs,  W.,  77,  106,  184 

Gilbaut,  H.,  116,  117,  120 

Gladstone,  J.  H.,  131,  142 

Gokun,  153,  156,  158,  165 

Goldsborough,  153 

Gouy,  G.,  189,  190,  191,  192,  206 

Graham,  Thomas,  9,  24,  31,  39,  40,  75, 

99,  145,  211,  212,  214,  220,  222,  223, 
224,   227,   231 

Groschuff,  E.,  227 
Guinchant,  116 
Guthrie,  F.,  130 

H 

Haber,  F.,  68 

Hamburger,  H.  J.,  94 

Hammarsten,  O.,  56 

Handovsky,  H.,  153,  154,  158,  170,  171, 

172,  256 
Hantzsch,  A.,  i 
Hardy,  W.  B.,  153,  169,  174 
Hartl,  F.,  139 
Hatschek,  E.,  5,  92,  138,  152,  153.  177, 

263 

Keen,  M.  P.  de,  127 
Heidenhain,  M.,  68 
Henri,  V.,  50,  153,  186,  187,  193,  200, 

201,  207 
Herz,  W.,  94 
Herzog,  R.  O.,  214,  218 
Heyer,  R.,  223 
Hibbert,  W.,  131,  142 
Hober,  R.,  54,  94,  225,  251 
Hoff,  J.  H.  van't,  68,  84,  128,  133,  142, 

144,  238,  253,  259 
Hoffmann,  F.,  105 
Hofmeister,  168 
Holde,  D.,  46,  103,  177,  178 
Hoppe-Seyler,  F.,  264 
Hiifner,  G.,  213,  233 
Hulett,  G.,  74 
Hulshof,  115 
Huth,  M.  E.,  112 


J 


Jahn,  St.,  192 
Johanott,  Phil.,  77 
Just,  J.,  103 


AUTHOR    INDEX 


269 


K 


Kasarnowski,  H.,  214 

Kassel,  R.,  146 

Kaufler,  F.,  95 

Kohnstamm,  77,  115 

Konowalow,  D.,  130,  141 

KSrner,  T.,  131 

Kossonogow,  J.  R.  von,  70 

Krafft,  F.,  129,  131,  142,  143,  224,  225, 

227 

Krulla,  R.,  62 
Kruyt,  H.  R.,  105,  106 
Kuenen,  J.  P.,  108 


Laar,  J.  J.  van,  232 

Lacqueur,  E.,  153,  169,  173,  174 

Lallemand,  106 

Lalon,  153 

Lebedew,  A.  von,  266 

Lecoq,  200,  201 

Lehmann,  O.,  57,  62,  64,  65,  68,  69,  79, 

187,  189,  191 
Lemoine,  G.,  104 
Levites,  S.  J.,  153,  154,  i55,  160,  161, 

I62,  164,  165,  173 
Lewis,  Wm.  C.  McC.,  92,  184,  185 
Liebermann,  L.,  131,  142,  257 
Liesegang,  R.,  93 
Lillie,  R.  S.,  144,  233,  234,  236,  243,  244, 

245,  246,  247,  248,  250,  251,  252, 

253,  256 
Linder,  S.  E.,  34,  124, 130, 131,  136,  141, 

181,  182,  216,  219,  220,  224,  228, 

229,  232 

Linebarger,  C.  E.,  131,  232 
Link,  62 

Ljubavin,  N.,  131 
Lobry  de  Bruyn,  C.  A.,  27,  139 
Lodge,  O.,  92 
Loeb,  J.,  86 
Loffler,  B.,  121 
Lorenz,  R.,  103 
Lottermoser,  A.,  131,  232 
Ludeking,  Chr.,  122,  126,  130 
Luppo-Cramer,  93,  98,  139 
Luther,  145 


M 


Malfitano,  G.,  131,  223,  233,  264 

Maltezos,  C.,  191 

Manea,  264 

Martici,  A.,  176 

Martin,  C.  J.,  232,  233,  242,  264,  265 

Massen,  Th.,  214,  218 

Maxwell,  68 

Mayer,  A.,  153 

Mayer,  H.,  28 

Mclntosh,  96 

Mecklenburg,  W.,  31,  189,  199,  206 

Mellor,  J.  W.,  94 

Mensbrugghe,  G.  van  der,  68,  191 

Metz,  G.  de,  117,  118 

Meyer,  W.,  131 

Michaelis,  L.,  35,  68,  86,  154 

Michel,  131 

Mittasch,  A..  106 

Molisch,  H.,  189,  191 

Moore,  B.,  232,  233,  241,  242,  243,  248 

Morris,  G.  H.,  131,  132 

Moruzzi,  G.,  131,  153 

Mostynski,  B.,  154 

Mouton,  H.,  n,  58,  64 

Miiller,  A.,  52,  153 

Miiller-Thurgau,  92 

Mylius,  F.,  227 

N 

Nernst,  W.,  94 

Neuberg,  C.,  98 

Neumann,  W.,  50,  56,  100,  174,  183,  225 

Noyes,  A.  A.,  50,  51 


Oettingen,  H.  von,  7 

Oholm,  L.  L.,  218 

Oker-Blom,  M.,  221,  222 

Oppenheimer,  94 

Ostwald,  Walther,  no,  138 

Ostwald,  Wilhelm,  3,  28,  60,  62,  63,  66, 
68,  73,  74,  Qi,  93,  94,  95,  103,  105, 
112,  121,  127,  138,  142,  145,  210, 

211,  212,  219,  231 

Ostwald,  Wolfgang,  23,  24,  25, 39, 47,  50, 
52,  68,  71,  82,  102,  107,  109,  no, 
in,  146,  148,  iS9,  X76,  177,  180, 
216,  247,  256 


270 


AUTHOR   INDEX 


Paal,  C.,  100 

Pappada,  N.,  130,  131 

Parker,  W.  H.,  232,  233,  241 

Paternd,  E.,  131 

Pauli,  Wolfgang,  40,  153,  163,  165,  170, 

171,    172,    173,    174,    220,    221,    248, 
251,   256,   257 

Pawlow,  P.,  92,  106,  108 

Pelet,  I..,  60 

Perrin,  J.,  30,  50,  51,  68,  187,  189,  191, 

192,  194,  198,  201,  202,  203,  204, 

205,  206,  208,  209 
Pfeffer,  W.,  232,  238,  239,  242,  253 
Pfenning,  F.,  225,  261 
Pickering,  S.  U.,  46,  177 
Picton,  H.,  34,  124,  131,  136,  141,  181, 

l82,    2l6,    219,    22O,    221,    224,    228, 
229,  232 

Pieroni,  A.,  93 
Pockels,  A.,  77,  181 
Posnjak,  G.,  104,  144 
Posternak,  S.,  251 
Potts,  H.  E.,  177 
Prange,  A.  J.,  134 
Preuner,  G.,  224,  225 
Procter,  H.,  173 
Pukall,  263 


Quincke,  G.,  52,  62,  63,  64, 107, 117, 124, 

126,  181,  182,  191,  263 
Quincke,  H.,  122 


Raffo,  M.,  93,  136 
Ramsden,  W.,  181 
Rankin,  106 
Raoult,  F.,  128 
Rayleigh,  Lord,  77,  85,  181 
RegSczy,  E.  von,  219,  221 
Reichel,  263 

Reid,  E.  W.,  233,  236,  241 
Reissig,  J.,  134 
Reynold,  77 
Richter,  B.  J.,  52,  59 
Ringer,  W.  E.,  154 


Roaf,  H.  E.,  232,  233,  242,  243,  245,  249 
Robertson,  T.  B.,  95,  131,  141,  142,  143, 

177 

Rodewald,  H.,  122,  123,  127,  143 
Rohland,  P.,  100 
Rontgen,  W.,  116 
Rose,  G.,  121 
Rossi,  G.,  153 
Rothe,  R.,  105 
Rothmund,  V.,  177,  179 
Rotinjanz,  L.,  105 
Rucker,  77 


Sabanejew,  A.,  130,  131,  142 

Sachs,  W.,  262 

Sackur,  O.,  153,  169,  173,  174 

Sahlbom,  N.,  15,  16,  154,  182,  229,  230 

Samec,  153 

Scala,  A.,  69,  72,  135,  137 

Scarpa,  O.,  153 

Schade,  H.,  36,  79,  94,  107,  108 

Scheffer,  G.,  214 

Schenk,  R.,  177,  184 

Schidrowitz,  P.,  153 

Schmidt,  C.,  62 

Schmidt,  W.,  122,  126,  264 

Schneider,  116 

Schneider,  J.,  103 

Schoep,  A.,  266 

Schorr,  K.,  153,  158 

Schroeder,  P.  von,  153,  154,  155,  156, 

159,  160,  161,  165,  166,  167,  168, 

169,  180,  256 
Schiitt,  77 

Seddig,  M.,  193,  198,  199 
Siedentopf,  H.,  27,  29,  58,  87,  88,  106, 

193,  194,  200 

Simon,  J.,  153,  156,  173,  177 
Smith,  A.,  105 
Smits,  A.,  129,  130,  131 
Smoluchowski,  M.  von,  206,  208,  209, 

216,  217,  218,  219,  261,  262 
Spring,  W.,  238 
Stahl,  F.,  262 
Starling,  E.  H.,  232,  233 
Stas,  97 
Stefan,  214 
Steiner,  H.,  156,  158,  159,  163.  164,  165 


AUTHOR   INDEX 


271 


Steinwehr,  H.  von,  92 

Stodel,  153 

Stokes,  G.,  198,  204,  205,  207,  209 

Stoltzenberg,  H.,  112 

Strassburger,  E.,  262 

Strutz,  A.,  143 

Suzuki,  S.,  227 

Svedberg,  The,  31,  76,  99,  *33,  *39,  189, 
190,  192,  193,  194,  195,  196,  197, 
198,  199,  201,  206,  207,  213,  214, 
216,  217,  218,  262 

T 

Tammann,  G.,  107,  130,  131,  141 
Teague,  O.,  224,  225 
Teletow,  J.,  93,  94 
Thomson,  J.  J.,  95,  201,  205 
Traube,  Moritz,  160 
Traube-Mengarini,  M.,  60,  69,  72,  135, 

137 

V 

Vanino,  L.,  134,  139 

Vegesack,  A.  von,  n,  140,  154,  155,  176, 
233,  235,  237,  238,  239,  240,  241, 
242,  243,  249,  258,  260,  261, 

Victorow,  C.,  154,  169,  181,  184 

Vignon,  L.,  214,  225 

Vogelsang,  H.,  62 

Voightlander,  F.,  212,  221 

Vries,  H.  de,  213 


W 


Waals,  van  der,  77,  115 

Wagner,  R.,  153 

Washburn,  G.  H.,  31 

Weber,  C.  O.,  153 

Wedekind,  E.,  98 

Weimarn,  P.  P.  von,  24,  33,  45,  54,  57, 
58,  59,  60,  61,  62,  63,  65,  89,  90,  91, 
92,  97,  99,  100,  101,  102,  105,  107, 
108,  no,  139 

Weinmayr,  J.,  180 

Wenzel,  93,  96,  97 

Whitney,  W.  R.,  131,  135,  220,  221,  263 

Wiener,  Chr.,  189,  190,  192,  196,  206 

Wigand,  A.,  106 

Wild,  A.,  60 

Wittich,  J.  von,  221 

Wolff,  L.  H.,  27,  139 

Wiillner,  A.,  128 


Zangger,  H.,  158,  232 

Zirkel,  F.,  62 

Zlobicki,  L.,  181,  182 

Zoja,  L.,  170,  173 

Zsigmondy,  R.,  27,  29,  30,  32,  42,  50,  52, 

58,  76,  101,  135,  139,  186,  187,  188, 

196,  210,  218,  223 


SUBJECT  INDEX 


Acid,  arsenious,  103 

Acids,  and  viscosity,  170,  171;  and 
Brownian  movement,  200;  and  os- 
motic pressure,  245 

Adsorption,  95,  179,  234,  266 

After-effects,  243 

Agar-agar,  viscosity  of,  159 

Age,  and  viscosity  of  suspensoids,  151; 
and  viscosity  of  emulsoids,  154 

Albumin,  viscosity  of,  171;  diffusion  of 
142,219,221,245;  osmotic  pressure 
of,  247,  249,  250;  filtrability  of,  265 

Alkalies,  and  viscosity,  170,  171;  and 
Brownian  movement,  200;  and  os- 
motic pressure,  245 

Alcohol,  and  viscosity  of  gelatine,  156 

Alcohol-sol,  41 

Allocolloids,  103 

Allotropism,  105 

Analysis,  colloid,  3,  12,  16;  capillary, 
IS,  229 

Arsenic  trisulphide,  220 

Arsenious  acid,  103 

Associated  liquids,  3,  103 

Avogadro's  constant  N,  208,  254 


B 


jff-Gelatine,  160 

Beer's  law,  i 

Behavior  of  electrified  sulphur,  69 

Benzopurpurin,  155;  osmotic  pressure 
of,  237 

Boiling  point,  128;  of  colloids,  130 

Boundaries,  21 

Brownian  movement,  186;  characteris- 
tics of,  1 86;  independence  of,  189; 
measurement  of,  192;  photogra- 
phy of,  193;  rotary  motion  of,  194; 
uniformity  of,  195;  velocity  of,  195; 


Svedberg's  law  of,  195;  and  specific 
surface,  196;  and  concentration  of 
dispersoid,  196;  and  viscosity,  197, 
198;  and  temperature,  198;  and 
added  substances,  199;  of  rubber, 
200;  and  electrical  charge,  201; 
and  gravity,  201;  and  Stokes*  law, 
204;  kinetic  theory  of,  205;  and 
molecular  weight,  209 


Calcium,  colloid,  197 

Capillary  analysis,  15,  229 

Capillary  phenomena,  72 

Capillary  pressure,  91 

Caramel,  240 

Casein,  142,  265 

Castor  oil,  178 

Catalysis,  94 

Cellulose,  176 

Chalk-sacs,  187 

Chamberland  filter,  263 

Chemical  energy,  and  specific  surface, 

93 

Chemical  heterogeneity,  22 
Cinematograph,  193 
Classification,  of  Zsigmondy,  29,  34;  of 

disperse  systems,  29,  33 
Closed  phase,  25 
Clotting,  40 
Coagulation,  40 
Collodion,  266 
Colloidality,  32 
Colloids,  recognition  of,  i ;  diffusion  of. 

9,  10,  142,  210,  214,  219,  221,  245; 

suspension  and  emulsion,  12;  lyo- 

philic  and  lyophobic,   13,  51,    52; 

coagulation  of,  13,  16;  viscosity  of, 

13;  electrical  properties  of,  14,  15; 

mutual  precipitation  of,  16;  as  dis- 


273 


274 


SUBJECT    INDEX 


Colloids,  cont'd— 

perse  heterogeneous  systems,  23; 
specific  surface  of,  28;  character- 
istics of,  39;  thermal  coefficient  of 
expansion  in,  126;  molecular  weight 
of,  140,  258;  surface  tension  of,  180; 
movement  in,  186;  diffusion  coeffi- 
cients of,  214;  osmotic  pressure  of, 
232;  filtration  of,  263,  265 

Colloid  analysis,  3;  special,  12;  outline 
of  methods  of,  16 

Colloid-chemical  nomenclature,  40 

Colloid  ice,  106 

Colloid  metals,  32,  197,  207,  255 

Colloid  solutions  (see  also  colloids  and 
colloid  systems),  4;  differentiation 
of,  from  true,  6,  9;  optical  proper- 
ties of,  6;  vapor  tension  of,  128; 
boiling  point  of,  130;  freezing 
point  of,  131;  saturation  in,  134. 

Colloid  state,  2,  14;  and  independence 
of  chemical  composition,  2;  theory 
of,  3,  21 ;  concept  of,  21,  99;  uni- 
versality of,  99 

Colloid  systems,  reversible  and  irrever- 
sible, 40;  volume  and  density, 
relations  in,  115,  120;  concentra- 
tion-variable and  complex,  136; 
supersaturation  in,  138;  viscosity 
of,  145;  dialysis  of,  222;  osmosis  of, 
231 

Complex  dispersoids,  36,  136 

Concentration,  and  disperse  systems, 
35 ;  effects  of,  47,  48,  49,  136 

Concentration- variable  systems,  36, 
136 

Condensation,  87;  theory  of,  88 

Congo  red,  239,  255,  260 

Copper  ferrocyanide,  240 

Cosmic  dust,  43 

Critical  mixtures,  37,  82,  105 

Crystal  formation,  59,  62 

Crystals,  liquid,  62 

Crystalline  constitution  of  colloids,  56 

Crystallinity,  concept  of,  56;  of  colloids, 
58;  theory,  58 

Crystallization,  56,  62 

Crystalloids,  characteristics  of,  39,  101 

Cube,  increase  in  surface  with  division 
of,  27 


Degree  of  dispersion,  4,  26;  and  diffu- 
sion velocity,  215 

Density  and  colloids,  1 24 

Determination  of  osmotic  pressure  of 
colloids,  258 

Dialysis,  10,  222,  223,  224 

Dialyzers,  224 

Dializability,  226 

Diffusion,  9;  apparatus,  10,  211;  coeffi- 
cients, 214;  of  colloids,  210,  211, 
217;  of  serum  albumin,  222 

Diffusibility,  210,  211 

Diminution  of  surface,  84 

Discontinuity  of  matter,  96 

Disintegration  tension,  82 

Disperse  phase  (see  also  disperse  sys- 
tems), 25 

Disperse  systems,  24,  32;  classification 
of,  29,  33,  42;  ionic  31;  of  gold,  32; 
concentration- variable,  35;  tem- 
perature-variable, 36;  complex,  36; 
solid  +  solid,  43;  solid  +  liquid, 
43;  solid  +  gas,  43;  effect  of  con- 
centration on,  45;  energetics  of,  66; 
effects  of  electrolytes  on,  170 

Dispersion,  4,  26,  31,  33,  77 

Dispersion  means,  25 

Dispersions,  31 

Dispersoids  (see  disperse  systems) 

Droplet  formation,  86 

Dust,  cosmic,  43 

Dyes,  surface  tension  of,  182;  dialy- 
sis of,  225 

Dynamic  surface  tension,  67,  185 


Egg-albumin   (see  also  albumin),   142, 

245 

Einstein-Smoluchowski  formula,  206 
Electrical  charge  (see  also  electrolytes) 

and  Brownian  movement,  201 
Electrical  energy  and  specific  surface,  92 
Electrical  fountain,  68 
Electrical  heart,  68 
Electrolytes,  and  viscosity  of  suspen- 

soids,  151;  and  gelatine,  156;  and 

colloid  diffusion,  219;  and  osmotic 

pressure  of  colloids,  245 


SUBJECT    INDEX 


275 


Electrophoresis,  16 

Emulsion  colloids  (see  emulsoids) 

Emulsions,  5 

Emulsoids,  12;  general  properties  of,  49, 
54,  124;  crystallinity  of,  64;  and 
suspensoids,  124;  viscosity  of,  153, 
159,  162,  164,  165,  169;  filtrability 
of,  266 

Energetics  of  dispersoids,  66 

Energy,  surface,  74,  77;  and  specific  sur- 
face, 92,  93,  97 

Expansive  surface  tension,  68,  69,  70 


Ferments,  94 

Filter  paper,  15;  and  capillary  analysis, 

230,  263 

Filters,  5;  ultra,  12,  263 
Filtrability,  263 
Filtration,  5,  263 
Fluid  mixtures,  critical,  105 
Fluorescence,  8 
Foams,  in 
Fog,  43 
Formation,  of  crystals,  59;  of  droplets, 

87 
Formula   of    Gibbs,    184;  of    Einstein- 

Smoluchowski,    206;  of    Svedberg, 

195 
Freezing  point,  128;  of  colloids,  131 


Gas  +  gas  dispersions,  43 

Gas  +  liquid  dispersions,  43 

Gas  +  solid  dispersions,  43 

Gelatine,  no,  139,  265;  viscosity  of  154, 
156,  159,  162,  167,  169;  surface 
tension  of,  182;  osmotic  pressure  of , 
236;  thermal  history  of,  242;  and 
acids  and  alkalies,  169,  245;  swell- 
ing of,  247,  256;  as  filter  medium, 
264 

Gelation,  40 

Gels,  24,  40;  permeability  of,  264 

Gibbs'  theorem,  184 

Glycogen,  142 

Gold,  32,  217,  220,  255 

Gravity  and  Brownian  movement,  201 


Gum  arabic,  142,  239,  240 

Gutta  percha,  177,  187,  191,  200,  204 


H 


Heat,  243,  262 

Hemoglobin,  241,  257 

Heterogeneity,  3,  21,  23;  concept  of,  21; 

chemical  and  physical,  22 
Hofmeister  series,  168 
Homogeneous  liquids,  3 
Hydrates,  39 
Hydrogels,  41 
Hydrosols,  41 
Hylotropic  changes,  3 
Hysteresis,  243 


Ice  colloids,  106 

Ice  cream,  no 

Increase  of  surface,  78 

Independence  of  Brownian  movement, 

189 
Instability  of  mechanical  suspensions,  5; 

of  osmotic  pressure  of  colloids,  235 
Internal  friction  (see  viscosity) 
Ionic  dispersoids,  31 
Ionic  series,  168;  and  osmotic  pressure 

of  colloids,  249 

Iron  hydroxide,  228,  229,  240,  265 
Iron  nitrate,  capillary  analysis  of,  230 
Isocolloids,  4,  8,  102;  of  water,  107 
Isodispersoids,  4,  8,  102,  107 
Isomeric  compounds,  3 


Kinematograph,  193 
Kinetic    theory   and    Brownian    move- 
ment, 204 


Latex,  200 

Law,  Beer's,  .1;  mass,  142;  Svedberg's, 
195;  Stokes',  204;  van't  Hoff's, 
259;  Wenzel's,  96 

Light,  and  Tyndall  effect  7,  8;  and  col- 
loid movement,  262 


276 


SUBJECT   INDEX 


Light  cone  of  Tyndall,  7,  8 

Liquid  crystals,  52 

Liquid  +  gas  dispersions,  49 

Liquid  -f-  liquid  dispersions,  48 

Liquid  +  solid  dispersions,  48 

Liquids,     heterogeneous     and     homo 

geneous,  3 

Lyophilic  colloids,  13,  52 
Lyophobic  colloids,  13,  52 


M 


Mass  law,  142 
Mastic,  204 
Masticized  rubber,  177 
Matter,  discontinuity  of,  96 
Measurement   of   osmotic   pressure  'of 

colloids,  233 
Mechanical    suspensions,    4;  instability 

of,S 

Membranes,  liquid,  87;  osmotic,  232 

Metameric  compounds,  3 

Metastyrol,  3,  104 

Methods  of  dialysis,  223;  of  studying 
diffusion,  211 

Microns,,  29 

Milk,  176,  186 

Minerals,  43 

Mixtures,  critical,  37,  82,  105 

Molar  surface  energy,  3 

Molecular  dispersions,  4,  31,  54;  viscos- 
ity of,  145 

Molecular  weight  of  colloids,  140,  142, 
258;  and  Brownian  movement,  209 

Moleculo-kinetic  theory  of  osmosis,  261 

Movement  in  colloids,  186 


N 


N.  (Avogadro's  number)  208,  254 
Nicol  prism,  8 

Night-blue,  140;  viscosity  of,  158,  163; 
osmotic  pressure  of,  243,  247,  249 
Nomenclature,  colloid-chemical,  40 
Normal  liquids,  4 
Nucleus,  138 

O 

Oil,  5,  178 

Optical  behavior  of  colloids,  6,  58 


Osmosis  (see  also  osmotic  pressure) ,  231; 
kinetic  theory  of,  261 

Osmotic  pressure,  of  benzopurpurin,  237; 
of  night-blue,  243,  247,  249;  of  gela- 
tine, 245,  250;  of  albumin,  247, 
249;  and  molecular  weight,  258 

Osmotic  pressure  of  colloids,  144,  232, 
249;  instability  of,  235;  and  pre- 
vious treatment,  236;  and  shaking, 
236;  and  stirring,  237;  and  time,  237; 
and  concentration,  238;  and  tem- 
perature, 242;  and  added  sub- 
stances, 244;  and  acids  and  alkalies, 
245;  theory  of,  253 


Particles,  size  of,  30 

Peptization,  40 

Permeability  of  niters,    263;    of    gels, 

264 

Phase  rule,  105 
Phases,    22;   closed,   25;    disperse,    25; 

physical  state  of,  42 
Phosphorus,  104 
Photography  of  Brownian   movement, 

iQ3 

Physical  heterogeneity,  21 
Platinum,  94,  197,  207 
Polydisperse  systems,  35,  54,  138 
Polydispersoids,  35,  54,  138 
Polymeric  compounds,  3 
Polysuspensoids,  54 
Pores  in  filters,  263 
Practical  introduction,  i 
Precipitation,  40 
Pressure,    capillary,    91;  osmotic,    232, 

233 

Prism,  Nicol,  8 

Protective  action,  5 

Proteins,  no,  139,  142,  265;  viscosity 
of,  154,  156,  159,  162,  167,  169; 
and  acids  and  alkalies,  170;  surface 
tension  of,  182;  diffusion  of,  222; 
osmotic  pressure  of,  236;  thermal 
history  of,  242;  and  acids  and  alka- 
lies, i6v),  245;  swelling  of,  247,  256; 
as  filter  medium,  264 

Pukall  filter,  263 


SUBJECT   INDEX 


277 


Radiant  energy,  97 

Radio-activity,  98 

Recognition  of  colloids,  i 

Reichel  niters,  263 

Reversible  systems,  40 

Rosin,  55,  179 

Rubber,  177,  187,  191;  masticized,  177; 

Brownian  movement  in,  200 
Rule  of  Gibbs,  184;   of   Einstein-Smu- 

luchowski,  206;  of  Svedberg,  195 


Sacs,  diffusion,  10;  chalk,  187 

Salol,  60 

Selenium,  104 

Serum-albumin,  viscosity  of,  171;  diffu- 
sion of,  222 

Silicic  acid,  as  filter  medium,  264 

Silver,  75,  190 

Size  of  particles,  30 

Smoke,  43 

Smoluchowski-Einstein  formula,  206 

Soap,  143,  257 

Solid  -f-  gas  dispersoids,  43 

Solid  +  liquid  dispersoids,  43 

Solid  -f-  solid  dispersoids,  43 

Sols,  24,  60;  alcohol,  41;  water,  41; 
sulphuric  acid,  41 

Solubility,  of  salol,  60;  of  silver  chlo- 
ride, 75 

Solutions,  true,  4,  54;  colloid,  4,  6,  9; 
molecular-disperse,  4,  6,  9;  optical 
behavior  of,  6;  supersaturated,  33, 
60;  vapor  tension  of,  128;  boiling 
point  of,  130;  freezing  point  of, 
131;  saturation  in  colloid,  134,  136; 
supersaturation  in  colloid,  138 

Solvates,  38,  54 

Special  colloid-chemistry,  115 

Specific  surface,  26,  72,  91,  92,  93;  of 
colloids,  29;  electrical  energy  and, 
92,  93;  chemical  energy  and,  93; 
and  Brownian  movement,  196 

State,  colloid  (see  also  colloid  state),  2. 
14;  theory  of,  3,  21 ;  concept  of,  99; 
universality  of,  99 

Stokes'  law,  32 


Strong  colloidality,  32 

Styrol,  104 

Submicrons,  29 

Sulphur,  60,  104,  105 

Supersaturated  solutions,  33 

Surface,  discontinuous  diminutions  in, 

84 

Surface  energy,  molar,  3 ;  of  first  order, 
66,  74;  of  second  order,  67,  74; 
and  other  energies,  71;  reciprocal 
effects  of  two  kinds  of,  79,  80 

Surface  increase,  78 

Surface  tension,  61;  dynamic,  67,  185; 
static,  67,  185;  negative,  68;  expan- 
sive, 68;  properties  of,  69,  70;  and 
specific  surface,  76;  of  colloids,  118; 
of  gelatine,  182;  of  dyes,  183 

Surfaces,  21,  27;  in  colloids,  29 

Suspension  colloids,  12;  general  prop- 
erties of,  49,  54,  124;  viscosity  of 
146,  151,  152 

Suspensions,    mechanical,   4,5;  colloid. 

49 

Suspensoids  (see  suspension  colloids) 
Svedberg's  law  of  Brownian  movement, 

195 

Swelling  of  gelatine,  247,  256 ' 

Systems,  22;  disperse,  24,  32;  classifi- 
cation of,  29,  33;  submolecularly 
disperse,  32;  supermolecularly  dis- 
perse, 32;  poly  disperse,  35;  con- 
centration-variable, 35,  136;  tem- 
perature-variable, 36;  colloid,  40, 
115,  120;  reversible  and  irrever- 
sible, 40;  dispersoid,  43 


Temperature,  and  viscosity,  164;  and 
Brownian  movement,  198;  and  hys- 
teresis, 242 

Tension,  surface,  61,  67,  79,  80;  dis- 
integration, 82 

Theorem  of  Gibbs,  184 

Theory  of  condensation,  88;  of  osmotic 
pressure  of  colloids,  253 

Thermal  coefficient,  and  expansion  in 
colloids,  136 

Thermal  history,  242 

Thorium  hydroxidesol,  240 


278 

Time,  and  osmotic  pressure  of  colloids, 

237 

Tobacco  smoke,  43 
Transition  phenomena,  39 
Transition  systems,  12,  39 
True  dispersions,  30 
True  solutions,  4,  54 
Tyndall  phenomenon,  7,  8 

U 

Ultrafiltration,  12,  263 

Ultramicrons,  29 

Universality  of  colloid  state,  99 


Van't  Hoff's  laws,  259 

Vapor  pressure,  128 

Vapor  tension,  128 

Vectorial  constitution  of  colloids,  56,  58, 
64 

Viscosimeter,  145 

Viscosity  (see  also  viscosity  of  emul- 
soids),  13;  of  colloid  systems,  145; 
of  molecular  dispersoids,  145;  of 
suspensoids,  146,  150,  151;  and 
electrolytes,  151;  mechanical  the- 
ory of,  in  suspensoids,  152;  of  emul- 


SUBJECT   INDEX 


soids,  153,  154,  158,  159,  161,  162, 
163,  164,  165,  167,  169,  171,  173, 
174,  175,  179;  of  gelatine,  154, 
156,  159;  of  benzopurpurin,  154; 
and  inoculation,  158;  of  night- 
blue,  158,  159;  of  agar-agar,  159; 
and  degree  of  dispersion,  175;  and 
type  of  disperse  phase,  179;  and 
Brownian  movement,  197,  198; 
and  character  of  dispersion  means, 
198 

Viscosity  of  emulsoids  (see  also  vis- 
cosity) and  age,  154;  and  electro- 
lytes, 154;  and  mechanical  treat- 
ment, 158;  and  concentration,  161; 
and  temperature,  164;  and  added 
substances,  165,  169,  171;  and 
non-electrolytes,  173;  and  electrical 
charge  of  dispersion  phase,  174. 

W 

Water,  isocolloids  of,  107 
Weak  colloidality,  32 
Wenzel's  law,  96 


Zoospores,  262 

Zsigmondy,  classification  of,  29 


YD  07397 


337588 


°,( 

, 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


